The Pennsylvania State University

The Graduate School

Department of Animal Science

FACTORS THAT AFFECT RUMEN FERMENTATION AND TOTAL

TRACT DIGESTION IN PRECISION FED DAIRY HEIFERS

A Dissertation in

Animal Science

by

Felipe Pino San Martin

2016 Felipe Pino San Martin

Submitted in Partial Fulfillment

of the Requirements

for the Degree of

Doctor of Philosophy

December 2016

The dissertation of Felipe Pino San Martin was reviewed and approved* by the following:

Arlyn J. Heinrichs

Professor of Dairy Science

Dissertation Advisor

Chair of Committee

Kevin D. Harvatine

Associate Professor of Nutritional Physiology

Chad Dechow

Associate Professor of Dairy Cattle Genetics

Gregory W. Roth

Professor of Agronomy

Terry D Etherton

Distinguished Professor of Animal Nutrition

Head of the Department of Animal Science

*Signatures are on file in the Graduate School

iii

ABSTRACT

In the last decade farmers and researchers have focused on nutritional methods

and management to improve feed efficiency in rearing dairy heifers. Precision feeding

has been an interesting alternative to traditional ad-libitum, high forage diets fed to

heifers. Precision feeding is an economical way to raise heifers that modifies

physiological and nutritional responses, making heifers more efficient without affecting

growth, first lactation milk production, or the animal in any way that we are aware. The

literature available has provided information about dietary crude protein, optimal N

intake, and diet forage-to-concentrate ratio (F:C) in precision feeding, but still more

information is necessary. In the present dissertation, three experiments were conducted to

evaluate the effect of starch, neutral detergent fiber (NDF) source, fiber digestibility, and

rate of passage in precision feeding dairy heifers.

The first experiment had two objectives: evaluate effects of starch concentration

on digestibility and rumen fermentation and compare two sources of trace minerals (TM;

inorganic, ITM, and organic, OTM, form) on digestibility and rumen fermentation. Eight

rumen cannulated dairy heifers (15.4 ± 0.8 mo of age and 438.31 ± 18.08 kg of body

weight) were subject to a split-plot, 4 × 4 Latin Square design with 19-d periods; 15 d

adaptation and 4 d sampling. The whole-plot factor was type of TM; organic as

proteinates (OTM) or inorganic sulfates (ITM), and the subplot was starch level (3.5,

12.9, 22.3, and 31.7%). Results of this experiment supported the hypothesis that the type

of TM affects rumen bacteria populations and produces responses in ruminal

iv

fermentation. Digestibility of dry matter (DM), NDF, acid detergent fiber (ADF),

hemicellulose, and starch was not affected by treatments. The OTM decreased rumen pH

and increased total volatile fatty acid (VFA) production and butyrate concentration. This

can be explained by the lower time consuming the ration with OTM, which led to a faster

fermentation. Also we hypothesize as the higher bioavailability of OTM suggests a faster

utilization of the TM and accelerated replication of ruminal micro-organisms, stimulating

ruminal fermentation and VFA production. Butyrate was also linearly increased as starch

level increased. In general, TM excretion was not affected by type of TM. Plasma TM

concentration was not different by treatment except for Mn, which was higher for OTM.

However, mineral intake was reduced in OTM, but blood concentration was not different

between TM types. These results suggest that OTM have higher TM absorption

compared with the ITM. On the other hand, urine and total manure excretion were higher

for ITM, suggesting that ITM stimulated water intake and produce more manure. In

summary, the type of TM affected rumen fermentation such that OTM was absorbed to a

greater extent than ITM, suggesting higher bioavailability for this form of TM.

The objective of the second experiment was to evaluate sorghum silage (SS),

including digestibility and fermentation parameters, in precision-fed dairy heifers. Eight

Holstein heifers (13.7 ± 0.6 mo of age and 364.8 ± 17.64 kg of body weight) fitted with

rumen cannulas were used in a replicated 4 × 4 Latin Square design; treatments were 4

levels of F:C (85:15, 75:25, 65:35, 55:45). When the concentrate proportion of the diet

increased, heifers tended to improve feed efficiency, primarily due to lower DM intake

(DMI) with the same average daily gain (ADG) over diets with a high proportion of

v

forage. Rumen pH was affected by F:C, decreasing as the proportion of concentrate

increased in the diet since heifers spent less time consuming feed. However, pH was

never lower than 5.7 in diets with F:C 55:45, and fiber digestibility was not affected.

Volatile fatty acid proportion was slightly influenced by treatment, where butyrate

increased as concentrate increased in the diet. Dry matter and starch digestibility were

affected by F:C and were improved in diets with more concentrate. Neutral detergent

fiber, ADF, and hemicellulose digestibility were not affected by F:C. Wet and dry feces

were reduced linearly as F:C decreased, but total manure was not affected by treatment

due to increased urine production on high concentrate diets. In the in situ analysis, corn

silage had a faster rate of digestion for DM and NDF than SS. This result suggests that

the overall digestion of SS was diminished, probably because of the high NDF. Brown

mid-rib SS can effectively be fed in precision diets for dairy heifers. Specifically in this

study, the 65:35 F:C presented better performance based on rumen fermentation, VFA,

rumen pH, digestibility, and feed efficiency.

The third experiment was conducted with the objective to compare ad-libitum vs.

precision feeding diets with two forages and different levels of NDF to evaluate rumen

fermentation, diet digestibility, feed efficiency, and digesta passage rate. Eight Holstein

heifers (18.4 ± 0.6 mo and 457.2 ± 27.29 kg BW) fitted with rumen cannulas were used

in a two-factor, split-plot, Latin Square design with 19-d periods, 14 d of adaptation and 5

d of sampling. The whole-plot factor was feeding system with ad-libitum or precision

feeding and 4 heifers in each plot. The subplot included 2 factors: forage quality (low

quality: grass hay, LFQ; high quality: corn silage, HFQ) and NDF content (high NDF,

vi

48 % HNDF; low NDF, 39.8 %, LNDF). In this study we showed that the reduction in

DMI for precision feeding diets improved feed efficiency in comparison with ad-libitum

diets for dairy heifers. We observed that HFQ diets increased DMI, resulting in altered

feed efficiency due to changes in intake based on fiber intake. Precision-fed diets resulted

in a lower minimum rumen pH than ad-libitum diets, but the amount of time spent at the

minimum pH was not great enough to reduce fiber digestion or rumen fermentation. Ad-

libitum diets resulted in lower mean pH than precision-fed diets, but the rumen pH was

more consistent throughout the day than in precision feeding, where rapid fermentation

resulted when heifers ate much of their daily diet within a small amount of time. This

effect was stronger when corn silage was the forage component of the diet. Also, we

observed that HNDF diets presented higher minimum pH, suggesting that the presence of

additional fiber stimulates rumination and buffers the rumen. Overall, VFA proportions

were not affected by the type of diet but were clearly modified by forage quality, where

grass hay diets had higher proportions of acetate and corn silage diets higher proportions

of propionate. Overall, apparent total tract digestibility was not affected by the type of

diet; however, DM digestibility increased with HFQ and decreased with HNDF level. In

situ digestibility was affected by forage quality and NDF level, where grass hay diets

resulted in a greater 48 h rumen degradation than corn silage. Rate of passage was not

affected by type of diet 22 h after feeding, but it was highly affected with the rumen at

maximum capacity, 3 to 4 h after feeding. In this study, ad-libitum diets had higher

passage rate than precision diets for the nutrients analyzed. Rate of digestion was affected

by forage quality in the post-feeding evaluation, indicating that corn silage diets had

vii

higher digestion rates than grass hay diets. This suggests that higher amounts of

indigestible NDF reduced the digestion capacity of the rumen. With the results obtained

in this study, we can state that the retention time for precision-fed diets was higher than

ad-libitum diets and could lead an increased rumen digestion of nutrients. Also, grass hay

diets had a higher retention time compared to corn silage diets. This effect was more

significant in the precision-fed heifers. In addition, fluid dilution rate was higher for the

ad-libitum diets. Grass hay diets presented a higher fluid dilution rate than corn silage-

based diets.

In summary, the three factors analyzed in this study affect ruminal fermentation,

rumen pH, nutrient digestion, and rate of passage, but the most important result was the

difference in feed efficiency presented in the precision feeding diets that could lead to a

reduction in the cost of raising dairy heifers.

Keywords: Heifers, precision feeding, starch, passage rate, digestibility, rumen

fermentation.

viii

TABLE OF CONTENTS

LIST OF FIGURES .................................................................................................... xi

LIST OF TABLES ..................................................................................................... xii

LIST OF ABBREVIATIONS ...................................................................................... xiii

ACKNOWLEDGEMENTS ........................................................................................ xiv

Chapter 1 ........................................................................................................ 1

Introduction ...........................................................................................................1

Chapter 2 ........................................................................................................ 5

Literature Review:Implications of Precision Feeding on Nutrient Digestion in Dairy

Heifers ...............................................................................................................................5

Traditional heifer feeding management and the impact on dairy farms ................... 5

Principles of precision feeding in dairy heifers ........................................................... 8

Improvement of feed efficiency ........................................................................................... 8

Physiological changes and metabolic adaptations to precision feeding ............................ 11

Reduction in metabolic nutrient cost ................................................................................ 11

Passage rate of nutrients and digestibility ......................................................................... 13

Factors affecting nutrient digestibility in precision feeding ..................................... 15

Starch Intake ...................................................................................................................... 16

Impact of F:C ...................................................................................................................... 19

Effect of NDF on digestion ................................................................................................. 21

Effect of DMI and passage rate on nutrient digestibility ................................................... 23

Conclusions ............................................................................................................... 26

ix

Chapter 3 ...................................................................................................... 39

Effect of trace minerals and starch on digestibility and rumen fermentation in diets

for dairy heifers .................................................................................................... 40

Abstract ..................................................................................................................... 40

Introduction .............................................................................................................. 40

Materials and Methods ............................................................................................. 41

Results and Discussion .............................................................................................. 43

Conclusion ................................................................................................................. 51

Chapter 4 ...................................................................................................... 54

Sorghum forage in precision-fed dairy heifer diets ................................................ 55

Abstract ..................................................................................................................... 55

Introduction .............................................................................................................. 55

Materials and Methods ............................................................................................. 56

Results and Discussion .............................................................................................. 58

Conclusion ................................................................................................................. 65

Chapter 5 ...................................................................................................... 67

Comparison of diet digestibility, rumen fermentation, rumen rate of passage, and

feed efficiency in dairy heifers fed ad-libitum versus precision rations with low and

high quality forages and 2 levels of neutral detergent fiber .................................... 67

Abstract ..................................................................................................................... 67

Introduction .............................................................................................................. 68

Materials and Methods ............................................................................................. 71

x

Animals, Treatments, and Experimental Design ................................................................ 71

Diets ................................................................................................................................... 71

Sample Collection and Analysis.......................................................................................... 72

Statistical Analysis .............................................................................................................. 76

Results and Discussion .............................................................................................. 77

Conclusions ............................................................................................................... 91

References ......................................................................................................... 106

Chapter 6 .................................................................................................... 119

Summary and conclusions .................................................................................. 119

xi

LIST OF FIGURES

Figure 5-1. Rumen pH and total VFA production over 24 h in ad-libitum (left column) vs.

precision-fed (right column) heifer diets with high forage quality (HFQ) or low

forage quality (LFQ) and high NDF (HNDF) or low NDF (LNDF). ............................... 103

Figure 5-2. Fermentation end products over 24 h in ad-libitum (left column) vs.

precision-fed (right column) heifer diets with high forage quality (HFQ) or low

forage quality (LFQ) and high NDF (HNDF) or low NDF (LNDF). ............................... 104

xii

LIST OF TABLES

Table 5-1. Ingredients and chemical composition of diets with high forage quality (HFQ)

or low forage quality (LFQ) and high NDF (HNDF) or low NDF (LNDF) .................... 92

Table 5-2. Body weight, intakes, and feed efficiency in ad-libitum (A-L) vs. precision-fed

(P-F) heifer diets with high forage quality (HFQ) or low forage quality (LFQ) and

high NDF (HNDF) or low NDF (LNDF) ......................................................................... 94

Table 5-3. Rumen pH, eating time, rate of eating, and VFA, in ad-libitum (A-L) vs.

precision-fed (P-F) heifer diets with high forage quality (HFQ) or low forage quality

(LFQ) and high NDF (HNDF) or low NDF (LNDF) ....................................................... 95

Table 5-4. Excretion parameters in ad-libitum (A-L) vs. precision-fed (P-F) heifer diets

with high forage quality (HFQ) or low forage quality (LFQ) and high NDF (HNDF)

or low NDF (LNDF) ........................................................................................................ 96

Table 5-5. Apparent total tract nutrient digestibility and in situ digestibility in ad-libitum

(A-L) vs. precision-fed (P-F) heifer diets with high forage quality (HFQ) or low

forage quality (LFQ) and high NDF (HNDF) or low NDF (LNDF)................................ 97

Table 5-6. Pre-feeding rumen digestion kinetics in ad-libitum (A-L) vs. precision-fed (P-

F) heifer diets with high forage quality (HFQ) or low forage quality (LFQ) and high

NDF (HNDF) or low NDF (LNDF) ................................................................................. 98

Table 5-7. Post-feeding rumen digestion kinetics in ad-libitum (A-F) vs. precision-fed (P-

F) heifer diets with high forage quality (HFQ) or low forage quality (LFQ) and high

NDF (HNDF) or low NDF (LNDF) ................................................................................. 100

Table 5-8. Fluid passage rate in ad-libitum (A-F) vs. precision-fed (P-F) heifer diets with

high forage quality (HFQ) or low forage quality (LFQ) and high NDF (HNDF) or

low NDF (LNDF) ............................................................................................................. 102

xiii

LIST OF ABBREVIATIONS

ADF Acid detergent fiber

ADG Average daily gain

BW Body weight

CP Crude protein

DM Dry matter

DMI Dry matter intake

DIM Days in milk

HC High concentrate

HFQ High forage quality

ITM Inorganic trace mineral

LFQ Low forage quality

NDF Neutral detergent fiber

OM Organic matter

OTM Organic trace mineral

RDP Rumen degradable protein

RUP Rumen undegradable protein

SD Standard deviation

TM Trace Mineral

TMR Total mixed ration

VFA Volatile fatty acid

xiv

ACKNOWLEDGEMENTS

I would like to thank all my family, for supporting me throughout all these years of

studies away from home. They are my motivation to continue every day and to

take new professional challenges in the future. I would like to thank Natalie, my wife who has

been a fundamental part of my life at Penn State. Without her it wouldn’t have been possible to

complete this stage of my education. I would like to thank my friends from Chile and from all

around the world for their help, support and good moments during these years.

I am really thankful to my advisor Dr. Heinrichs who was more than an advisor, he was part of

my family here in the US. He gave me all the support to develop my projects during these years.

Also I want to thank my committee members, Dr. Harvatine, Dr. Roth and Dr. Dechow for the

help and guidance during my PhD program and beyond. Thanks to all the undergraduates and

collaborators in my research projects, without them it wouldn’t be possible to finish all the studies

that we did in this period.

Special thanks to Susan Strauch for her help and support taking care of Santiago during my last

year at Penn State, without her it wouldn’t have been possible to finish my PhD.

1

Chapter 1

Introduction

Finding strategies to raise dairy heifers economically and efficiently is one of the

most important topics for dairy farms. The profitability and performance of a dairy farm

will depend on the efficiency of managements that maximize milk production while using

resources wisely. Currently, a lot of effort is focused in lactating dairy cattle and most of

the time, raising dairy heifers is not a priority for dairy farmers. However this situation is

contradictory, because heifers are the 2nd

largest contributor to whole farm expenses

(Tozer and Heinrichs, 2001).

This situation deserves more concern and dedication by dairy farms and

researchers, as opportunities to reduce whole farm expenses by reducing the cost of

rearing dairy heifers exist. The management cost of raising heifers until initiation of

lactation can be reduced by reducing expenses and improving growth rates, minimizing

the time that heifers are unproductive (Hoffman et al., 1996).

Feed cost represents 60-65% of the total expenses associated with dairy heifer

growth until lactation (Gabler et al., 2000). Therefore, it is important to reduce feed cost,

improve nutritional management and increment profitability in rearing heifers. Feeding

practices that enhance profitability, reduce nutrient losses and produce physiological

changes that improve efficiency are required for dairy heifers.

Traditionally dairy heifers nutrition is based on low quality forages from weaning

until parturition (Heinrichs, 1996). However, based on heifer requirements this traditional

2

feed is inefficient if we consider energy and protein nutrition, (Moody et al., 2007;

Zanton and Heinrichs, 2009). In the last decade, research has focused on nutritional

strategies that increase efficiency of growing dairy heifers. Thus, heifer diets that contain

higher nutrient density and highly digestible feeds have been used to improve feed

efficiency (Hoffman et al., 1996; Loerch, 1990). Improvements in feed efficiency

involve less use of feeds, greater ADG and less waste of nutrients that will be lead in a

reduction of expenses and greater profitability. Also, when feeding nutrient dense diets,

heifers reduce DMI, and decrease the amount of manure output (Moody et al., 2007). In

addition, heifers require less energy for digestion and hence, energy used for growth is

enhanced when feeding highly digestible diets (Zanton and Heinrichs, 2007).

Limiting the amount of DMI without affecting energy and protein supply is

considered limit feeding (Hoffman et al., 1996; Loerch, 1990; Zanton and Heinrichs,

2008). When heifers are limit fed with isonitrogenous and isocaloric diets similar growth

and lactation performance are observed compared to ad-libitum fed heifers (Lascano et

al., 2009; Zanton and Heinrichs, 2007). Thus, this feeding system allows normal heifer

growth, without affecting the mature body size or further milk production (Zanton and

Heinrichs, 2007). Also, studies showed an increase in feed efficiency. Lately some

studies have evaluated N efficiency and the effect of F:C ratio in limit feeding diets, but

digestibility, rumen fermentation, and fiber degradability data are needed to understand

the whole scenario in precision feeding dairy heifers. Precision feeding systems involve

decreased DMI using highly digestible nutrients and feeding high energy dense diets,

according to the requirements. Although precision feeding diets has been commonly

used lately in research and farms, it still generates some concern, principally because of

3

the high proportion of concentrates that could lead to low rumen pH due to rapid

fermentation.

There is limited information in the scientific literature that approach precision

feeding system to dairy heifers. For that reason, the purpose of this research was to

evaluate nutrient utilization in precision fed dairy heifer, and nutritional implications,

including fiber digestibility, rumen fermentation, and rate of passage of nutrients. Thus,

this research expands our understanding of precision feeding diets and provides changes

in the requirements for dairy heifers precision-fed during the growing period.

4

References

Gabler, M. T., P. R. Tozer, and A. J. Heinrichs. 2000. Development of a Cost Analysis

Spreadsheet for Calculating the Costs to Raise a Replacement Dairy Heifer1. J

Dairy Sci 83:1104-1109.

Heinrichs, A. J. 1996. Nutrition and management of replacement cattle. Animal Feed

Science and Technology 59:155-166.

Hoffman, P. C., N. M. Brehm, S. G. Price, and A. Prill-Adams. 1996. Effect of

Accelerated Postpubertal Growth and Early Calving on Lactation Performance of

Primiparous Holstein Heifers. J Dairy Sci 79:2024-2031.

Lascano, G. J., G. I. Zanton, F. X. Suarez-Mena, and A. J. Heinrichs. 2009. Effect of

limit feeding high- and low-concentrate diets with Saccharomyces cerevisiae on

digestibility and on dairy heifer growth and first-lactation performance1. J Dairy

Sci 92:5100-5110.

Loerch, S. C. 1990. Effects of feeding growing cattle high-concentrate diets at a restricted

intake on feedlot performance. 68:3086-3095.

Moody, M. L., G. I. Zanton, J. M. Daubert, and A. J. Heinrichs. 2007. Nutrient utilization

of differing forage-to-concentrate ratios by growing Holstein heifers. J Dairy Sci

90:5580-5586.

Tozer, P. R., and A. J. Heinrichs. 2001. What Affects the Costs of Raising Replacement

Dairy Heifers: A Multiple-Component Analysis1. J Dairy Sci 84:1836-1844.

Zanton, G. I., and A. J. Heinrichs. 2007. The effects of controlled feeding of a high-

forage or high-concentrate ration on Heifer growth and first-lactation milk

production. J Dairy Sci 90:3388-3396.

Zanton, G. I., and A. J. Heinrichs. 2009. Digestion and nitrogen utilization in dairy

heifers limit-fed a low or high forage ration at four levels of nitrogen intake. J

Dairy Sci 92:2078-2094.

Zanton, G., and J. Heinrichs. 2008. Precision feeding dairy heifers: strategies and

recommendations. College of Agricultural Sciences, DAS:08-130.

5

Chapter 2

Literature Review: Implications of Precision Feeding on Nutrient Digestion in

Dairy Heifers

Traditional heifer feeding management and the impact on dairy farms

It is known that dairy heifers are the future of herd milk production, but not

enough investigation has been done in this area. Adequate heifer nutrition is key for

optimization of body weight gain before calving, for proper development of the

mammary gland, and for future milk production. Because heifers are in a unproductive

period, many farmers do not take time to focus the appropriate management of these

animals. In addition, many farmers are not aware of the impact of that heifer nutrition can

have on future production of dairy cows. Compared to dairy cow nutrition research, very

little work has been done in dairy heifers over the past 50 years, and the majority of dairy

replacement research is focused on colostrum and calf nutrition (Eastridge, 2006). Even

though dairy cows provide the major farm income, heifers represent the second or third

largest cost towards the production of milk (Harsh et al., 2001; Tozer and Heinrichs,

2001) and also comprise a large proportion of animals in the farm inventory. Changes in

heifer management can impact farm profitability and productivity (Hutjens, 2004; Zanton

and Heinrichs, 2005). These features make heifer growth, nutrition, and reproduction

interesting areas for research; where outcomes could reduce expenses in raising dairy

heifers.

The cost of raising heifers often represents 15 to 20% of the total annual expenses

in a dairy farm, and nutrition is 60 to 70% of this cost (Gabler et al., 2000; Harsh et al.,

6

2001). Traditionally, dairy heifers are fed ad-libitum with high-forage, low-energy diets

to meet their requirements; however, the amount of fiber consumed limits DMI. A

tremendous disadvantage of this ad-libitum system is that heifers select feedstuffs in the

ration, providing a non-homogeneous intake by a group of heifers, which could

potentially affect rumen health. Also, heifers are physiologically inefficient in relation to

the digestion and utilization of forages to meet their requirements, therefore feeding ad-

libitum diets increases waste of nutrients, contributing negatively to the environmental

efficiencies in a dairy farm (Zanton and Heinrichs, 2009b). One of the main reproductive

objectives of dairy farms to reduce expenses is to decrease the age at first calving to 22 to

23 mo (Heinrichs, 1993); however, to achieve this goal it has been shown to be necessary

to improve growth performance (Hoffman et al., 2007). Tozer and Heinrichs (2001)

estimated that when the age at first calving is reduced from 25 to 21 mo (by increasing

diet energy density), the cost of raising dairy heifers is reduced by 18%. However,

increased energy in the ration is not the only change that it is necessary to achieve this

goal. Several studies show that when higher energy diets are offered, heifers increase pre-

puberty growth rate, but when animals are over conditioned has negative effects on time

to conception, age at first calving, and difficulties at first calving, while first lactation

milk yield is reduced (Little and Kay, 1979; Foldager and Sejrsen, 1991).

Little and Kay (1979) observed milk yield decreased between 15 to 48% in first

lactation heifers when high energy diets were used to increase ADG without controlling

DMI, and Foldager and Sejrsen (1991) reported a 10 to 25% reduction in milk yield when

pre-pubertal growth rate increased 0.6 kg/d over the control ADG (0.8 kg/d). When

switching heifer diets from high forage to high concentrate (HC) by using a readily

7

digestible carbohydrate (ground wheat), Tremere et al. (1968) observed accumulation of

rumen lactic acid, a decline in rumen pH under 5.0, and a shift in rumen fermentation that

reduced fiber digestion and VFA concentration. Tajima et al. (2001) agreed with this

information, while observing a depression in abundance of cellulolytic bacteria when

feeding HC diets. Calsamiglia et al. (2008) also stated rumen pH affects the pattern of

VFA production and true digestion, where HC diets that reduce rumen pH, reduce the

concentration of acetate and butyrate produced and also reduce OM and NDF

digestibility, reducing efficiency of nutrient utilization.

However, as a reduction in the age of calving is desired, researchers have been

recently investigating how energy and DMI affect heifer growth without affecting

production potential, health, or welfare (Hoffman et al., 2007; Moody et al., 2007;

Lascano and Heinrichs, 2009; Zanton and Heinrichs, 2009b; Pino and Heinrichs, 2016).

Recent investigations have focused on nutritional changes that modify and increase feed

efficiency of dairy heifers using energy dense diets, increasing energy density while

reducing DMI (limit-feeding), without affecting rumen health and further milk

production (Zanton and Heinrichs, 2005; Hoffman et al., 2007; Hall, 2008; Zanton and

Heinrichs, 2009b). A limit-fed, energy dense diet that provides nutrients required for

optimal growth in dairy heifers is a feeding system called precision feeding. Precision

diets provide energy and protein to meet the requirements, reduce growth energy

expenses, and improve feed efficiency in dairy heifers (Zanton and Heinrichs, 2009b). It

has been demonstrated that this feeding system improves feed efficiency, reduces nutrient

losses, and decreases manure production (Hoffman et al., 2007, Moody et al., 2007;

Lascano et al., 2009; Zanton and Heinrichs, 2009b; Pino and Heinrichs, 2016). Zanton

8

and Heinrichs (2008a) observed that in precision feeding systems each kg of reduction in

DMI, manure output decreased 2.6 kg. The reduction in manure reduces labor and

expenses related to the management of manure and its disposal, and what is most

important is that nutrient losses are also reduced.

Principles of precision feeding in dairy heifers

As mentioned, feed cost is the principal expense associated with rearing dairy

heifers (Gabler et al., 2000). To substantially reduce this cost, a reduction in the age at

first calving through increased ADG is necessary. However, ration costs need to be

reduced also. The best method to reduce feeding costs is optimizing nutrient intake, by

feeding animals to meet their requirements (precision feeding). This way, nutritional

requirements are covered while nutrient losses are minimized (Hoffman et al., 2007;

Zanton and Heinrichs, 2008a). Thus, precision feeding improves feed efficiency through

a reduction in DMI, while keeping a constant ADG (Loerch, 1990; Galyean et al., 1999;

Hoffman et al., 2007; Zanton and Heinrichs, 2008a). Precision feeding has also been

reported in beef cattle as the most traditional way to reduce expenses (Koch et al., 1963;

Loerch, 1990; Galyean et al., 1999).

Improvement of feed efficiency

Feed efficiency can be affected by several factors such as genetics, nutrient

digestibility, forage quality, growth rate, age, body condition, gestational stage,

temperature, and level of exercise, among others (Zanton and Heinrichs, 2008b). Genetic

9

selection of cows towards greater milk production has also increased average body size,

simultaneously increasing DMI, energy, protein, and other nutrient requirements (Gabler

et al., 2000). However, in recent years feed efficiency has been included in the

parameters of bull selection, with a heritability of 0.37 (Van Arendonk et al., 1991). In

beef cattle, feed efficiency has been selected for longer time and has a higher heritability

than in dairy cattle (Arthur et al., 2001).

The effect of DMI on feed efficiency has been extensively studied in dairy

heifers. Traditional low-energy, high-forage diets limit energy intake because of the high

fiber content (NRC, 2001) and prevent fat deposition in the pre-calving heifers. However,

feeding dairy heifers with NRC (2001) recommendations greatly exceeds the optimum

ADG, generating over-conditioned dairy heifers (Hoffman et al., 2007; Anderson et al.,

2015; Akins, 2016). Limiting feed intake and providing nutrient dense diets that cover

requirements is another way to improve feed efficiency (Loerch, 1990; Hoffman et al.,

2007). In dry cows, limiting feed intake improves digestibility of DM and reduces feed

cost (Driedger and Loerch, 1999). Similar observations were reported in dairy heifers,

where reducing feed intake controlled growth rates without affecting first lactation milk

yield (Lammers et al., 1999). This feeding system has been successfully used in beef

cows (Loerch, 1996), ewes (Susin et al., 1995), and beef heifers (Wertz et al., 2001)

without affecting production or animal performance.

The relationship between energy intake and energy retention is not linear. Thus,

maximum feed efficiency does not occur at maximum energy intake (Ferrell and Jenkins,

1998). This is the main justification of limit feeding, where feed efficiency is improved

by managing nutrient utilization (Loerch, 1990; Galyean et al., 1999).

10

The metabolic nutrient cost of digestion in animals is higher as DMI increases.

Nutrient digestion and absorption carried out by the gastrointestinal (GI) tract requires

intense oxidative metabolism and uses a great portion of dietary energy. Remaining

energy will be used for maintenance, growth, and productivity. Importantly, the GI tract,

liver, spleen, and pancreas together use around 40 to 50% of body oxygen consumption.

As the amount of nutrients to digest is larger, metabolic activity and oxygen consumption

will increase, increasing nutrient utilization (Huntington and Reynolds, 1983; Reynolds et

al., 1991b).

In growing steers feed efficiency improved by 30% when intake was reduced 20

or 30% from ad libitum diets (Loerch, 1990). Importantly all diets kept the same net

energy for maintenance and growth, and the animals maintained the same ADG. The

improvement in feed efficiency observed when reducing DMI is explained by a reduction

in rumen passage rate (Tamminga et al., 1979), allowing increased digestibility (Loerch,

1990). Increased digestibility is accompanied by reduction in feed waste (Hoffman et al.,

2007); and reduced DMI is accompanied by a reduction in gut and liver size (Reynolds et

al., 1991b) that reduces energy requirements for maintenance and increases energy

available for growth (Loerch, 1990; Hoffman et al., 2007). These observations have been

found in heifers fed energy dense diets; as DMI decreased, feed efficiency was improved

(Wertz et al., 2001). A 10 or 20% reduction in intake allowance reduced manure output

by 12.9 and 34.6%, respectively, while feed efficiency improved 23.7 and 28.9%

compared to ad-libitum diets (Hoffman et al., 2007).

Thus, by restricting DMI passage rate is reduced, while nutrient digestion and

absorption are increased, and nutrient waste and manure are reduced. Overall, dairy

11

heifer feed efficiency is improved without negative effects on growth, health, or future

milk production (Zanton and Heinrichs, 2009b).

Physiological changes and metabolic adaptations to precision feeding

Precision feeding involves the correct use of the nutrients, principally highly

digestible nutrients, to provide a controlled and optimum ADG (Zanton and Heinrichs,

2005) and supply energy above maintenance to stimulate growth. To achieve this target,

precision feeding systems reduce feed intake but use nutrient dense diets reduce

metabolic nutrient costs (Reynolds et al., 1991a,b).

Reduction in metabolic nutrient cost

Limit feeding of dietary DM reduces the cost of energy used for nutrient

digestion. Because energy consumption is based on requirements, fat deposition is

limited by not overfeeding, and energy is partitioned to maintenance and growth, thereby

increasing overall metabolic efficiency (Jarrett et al., 1976; Owens et al., 1995; Harsh et

al., 2001). Energy dense diets (those with a higher proportion of concentrates), result in

higher retention of energy in the tissues and reduced heat energy production. This was

observed by Reynolds et al., (1991a,b) when comparing beef heifers fed a constant level

of ME using 2 diets; 75% concentrate or 25 % concentrate. They also observed that

heifers that received high concentrate diets had increased DM digestibility and feed

efficiency.

With a reduction in feed intake, nutrients are partitioned to metabolic and

biological processes required for growth and maintenance (McLeod et al., 2007). Also,

12

fat storage is limited, which is beneficial as it is known that increased fat and growth

decrease first lactation milk production (Zanton and Heinrichs, 2005). Improvement in

feed efficiency is also achieved because as DMI decreases, visceral size and weight is

reduced, and therefore energy used by the portal drained viscera for digestion processes is

also reduced (Ferrell et al., 1986; Ferrell and Jenkins, 1998; McLeod and Baldwin, 2000;

McLeod et al., 2007). The metabolic energy demand of the GI tract for digestion and

absorption is very high; therefore, if GI tract size is reduced, oxygen and energy

consumption by the portal drained viscera will be lower and more dietary energy will be

available for other tissues (Reynolds et al., 1991b). Also Reynolds (2002) found that

splanchnic tissues are responsible for 40 to 50% of total body oxygen consumption;

therefore, heat increment and energy utilization will increase as splanchnic tissue mass is

greater. In sheep, higher DMI was associated with greater weight and size of internal

organs, where internal organs weighed up to 34% of BW (Colucci et al., 1989; Burrin et

al., 1990). When feeding isoenergetic diets but with different F:C, higher intake in the

high forage diets affected internal organ mass, increasing the energy used by the GI tract

(McLeod and Baldwin, 2000). Also, in restricted fed lambs, organ mass was reduced as

compared to ad-libitum, increasing feed efficiency (Fluharty and McClure, 1997). In beef

heifers, as energy density increases (higher proportion of concentrates), tissue energy

retention is higher and heat increment due to digestion is reduced (Reynolds et al.,

1991a,b).

13

Passage rate of nutrients and digestibility

As is described by Tamminga et al. (1979) and Wertz et al. (2001), limit feeding

diets decrease rumen passage rate and increase diet retention time in the rumen (Lascano

and Heinrichs, 2009). This allows increased exposure of diet components to rumen

microorganisms, and therefore, increased rumen digestion and fermentation (Tamminga

et al., 1979; Firkins et al., 1986; Merchen et al., 1986; Dijkstra, 1992). Bell (1971) stated

that gut capacity remains a constant fraction of BW, but as BW increases some metabolic

activities decrease, affecting efficiency. Clauss and Hummel (2005) stated that at high

intakes the ratio of organ and gut surface to gut volume remains constant, but when

intake decreases volume will decrease and the ratio will be greater. In this scenario,

nutrients are retained for a longer time in the rumen, allowing more extensive degradation

and utilization by rumen microbes. In addition, the surface area for digestion and

absorption in the gut becomes proportionally higher, improving the interaction between

nutrients and enzymes. This situation makes digestion and absorption more efficient.

Firkins et al. (1986) and Merchen et al. (1986) observed that as DMI increases,

rumen digestibility decreases and the pattern of ruminal fermentation changes. In diets

where intake was reduced by changes in energy content (energy dense diets), propionate

increased at the expense of acetate. Also, with HC diets microbial N and the efficiency of

microbial protein synthesis increases (Merchen et al., 1986; Colucci et al., 1990; Zanton

and Heinrichs, 2008a). In dairy heifers, the reduction in passage rate will reduce

microbial protein flow to the small intestine, which is compensated by higher protein

digestion and N retention (Zanton and Heinrichs, 2008a).

14

Changes in DM digestibility are variable and depend on diet F:C (Tyrrell and

Moe, 1975). Experiments in sheep and beef cattle, Colucci et al., (1990) compared

different F:C and observed that nutrient digestibility increases as concentrates increase in

the diet. Pino and Heinrichs (2016b) and Lascano and Heinrichs (2011) observed the

same effect in dairy heifers. Importantly, increased nutrient digestibility is observed with

a reduction in total intake. In the last decade many studies have proven that HC diets that

reduce DMI do not affect rumen pH, rumen health, and fiber digestion in precision-fed

heifers (Moody et al., 2007; Lascano and Heinrichs, 2009; Lascano et al., 2014; Ding et

al., 2015; Pino and Heinrichs, 2016). In addition, maximum rumen pH was higher for

precision-fed dairy heifers than ad-libitum systems (Chapter 5). Increased digestibility is

accompanied by a reduction in methane emissions and output of feces and urine, and

therefore reduced nutrient loss (Reynolds et al., 1991b). Reduced manure production

decreases farm expenses associated with manure management.

In beef and dairy heifers, N intake is higher when feeding low concentrate diets;

however, N efficiency and retention is greater with HC diets (Reynolds et al., 1991a;

Zanton and Heinrichs, 2008a; Lascano and Heinrichs, 2011). Also, fecal DM, urine, and

urinary N excretion were mostly lower when feeding HC diets. Amino acids, urea N, and

glucose released by splanchnic tissues to peripheral organs was greater when animals

received HC diets, making more nutrients available to other tissues and for growth in the

case of heifers (Huntington et al., 1996; Zanton and Heinrichs, 2007). Also, Zanton and

Heinrichs (2007) found that maximum N efficiency was at 1.8 g N intake/kg BW0.75

in

dairy heifers and concluded that N use is more efficient when feeding HC diets.

15

Factors affecting nutrient digestibility in precision feeding

An important feature of nutrient digestion in ruminants is that enzymatic

hydrolysis is the principal mechanism of digestion (Van Soest, 1994). Ruminants and

rumen microbes coexist through a symbiotic relationship, where dietary components are

utilized and fermented by rumen microorganisms, and their end products are nutrients

used by the ruminant (Van Soest, 1994). Even though a high proportion of concentrates

are used in the dairy industry, forages still are the principle source of nutrients for

ruminant production systems. However, low DM digestibility due to plant cell walls is

associated with limited energy in forage-based diets. This low digestion of nutrients leads

to a less efficient animal in the dairy industry (Jung, 1989; Galyean and Goetsch, 1993).

There are many factors that affect nutrient digestibility in ruminants. Some of the

most important are: chemical composition of feedstuffs, vegetative stage of the plants or

grains at harvest, type of grain processing, dietary load, rate of digestion, passage rate,

nutrient interactions, rate of fermentation, particle size, and F:C among others (Jung,

1989; Van Soest, 1994). The interaction of these factors can modify digestion of

nutrients; even when comparing diets with equal nutritional value, digestibility of

feedstuffs could be very different. This review will focus on discussing some important

factors that are not well studied in precision feeding diets for dairy heifers and are in

close relation with the studies done recently. The factors analyzed below are starch

intake, F:C, NDF, DMI, and passage rate.

16

Starch Intake

Carbohydrates are the principal source of energy for maintenance, growth, and

productivity in animals (Armstrong, 1965). Houseknecht et al. (1988) observed that beef

heifers fed low energy-low starch diets presented lower plasma insulin-like growth factor

and reduced growth, ADG, and backfat and ribeye area. However, feeding highly

digestible nutrients, specifically rapidly fermentable non-structural carbohydrates, to

dairy heifers is still a concern for dairy farmers and researchers (Nocek, 1997;

Kmicikewycz and Heinrichs, 2014). High starch diets are often associated with changes

in the rumen bacteria that shift toward amylolytic microbes, thus decreasing rumen fiber

digestion (Tremere et al., 1968; Hoover, 1986; Nocek, 1997). Precision feeding diets in

dairy heifers contain highly digestible nutrients to reduce DMI, increase digestibility, and

improve feed efficiency (Lascano and Heinrichs, 2009; Zanton and Heinrichs, 2009b).

However, precision-fed dairy heifers do not present sub-acute ruminal acidosis (SARA)

or depression of DM digestibility. High intake cows are exposed to large amounts of

highly fermentable carbohydrates, often in rations that are fiber deficient. Fiber deficient

diets reduce rumination and salivation; therefore, flow of salivary buffer to the rumen is

decreased and susceptibility for SARA increases (Kmicikewycz and Heinrichs, 2014). In

cows, increments of readily fermentable carbohydrates depress fiber digestion due to a

marked preference of microbes for carbohydrates instead of fiber; this leads to a decrease

in rumen pH and a reduction of cellulolytic microorganisms, and therefore of fiber

digestion (Hoover, 1986). However, DMI and the total amount of highly digestible

carbohydrates (principally starch that is more associated with SARA) in precision-fed

dairy heifers is reduced to cover requirements; hence rumen pH does not decrease to

17

levels that affect rumen digestion (Moody et al., 2007; Lascano and Heinrichs, 2009;

Zanton and Heinrichs, 2009b; Pino and Heinrichs, 2016).

It is important to state that dietary starch concentration can affect rumen pH and

fiber digestibility if the inclusion of the starch is abrupt and without an adaptation period

(Tremere et al., 1968; Hoover, 1986). This occurs due to the rapid increment of

Streptococcus sp. and the large proportion of lactic acid released in the rumen within a

couple of h after feeding (Cerrilla and Martinez, 2003). However, if the starch increment

is gradual over time and the microbial population shifts to more amylolytic bacteria, it is

possible to expect no changes in fiber nor DM digestion and, importantly, no ruminal

acidosis (Tremere et al., 1968).

Starch digestion per se depends on different factors, for example: starch source

and type, DMI, dietary composition, processing, and adaptation of microbes (Huntington,

1997). Different grains have a different digestion capacity and time of rumen

fermentation (Church, 1988; Huntington, 1997). In general, ground grains have higher

digestibilities than whole (Moe and Tyrrell, 1977) and flaked corn is more rapidly

fermentable in the rumen than rolled corn (Schuh et al., 1970). Effective starch

degradability is also different between grains. Steam-flaked corn and sorghum have the

highest starch degradability, while wheat or barley have the highest degradability when

the grain is ground (Offner et al., 2003). Barley, wheat, and oats have a high fraction of

immediately soluble starch in comparison to corn. These characteristics affect rumen

fermentation and rumen pH and modify nutrient digestion (Offner et al., 2003).

Pino and Heinrichs (2016) evaluated the effect of 4 different starch concentrations

(3.5, 13, 22.3, and 32%) in precision-fed dairy heifers. Mean pH was higher for the 2

18

highest starch concentrations, and pH never decreased under 5.7. Dry matter digestibility

was not depressed by any treatment, and DM and starch digestibility were higher at

higher dietary starch concentrations. Also, hemicellulose digestion decreased as starch

intake was higher; however, Lascano et al. (2012) observed a tendency for hemicellulose

digestion to increase with higher starch in the diet. In addition, Lascano and Heinrichs

(2009) and Moody et al. (2007) reported a lower minimum rumen pH in precision-fed

dairy heifers fed different F:C, but in none of them was DM or fiber digestibility

decreased when compared to traditional ad-libitum diets. Also, Lascano and Heinrichs

(2011) observed higher DM, NDF, and starch digestibility in HC-high starch diets.

Addition of feed additives can modify nutrient digestion. Lascano et al. (2012)

reported that regardless of starch concentration (16.7 vs. 28%), DM, OM, NDF, and

starch digestibility were not different as the dose of dietary yeast (Saccharomyces

cerevisiae) increased; however, DM, OM, hemicellulose, NDF, and ADF digestibility

increased quadratically with increasing dose of yeast.

Colucci et al. (1989) evaluated the effect of intake and dietary concentrate on

digestibility in cows and sheep. Here, DM, starch, and hemicellulose digestibility

increased with higher dietary starch in sheep, and DM digestibility increased in cows

regardless of DMI (10, 22, and 32.4% starch). When comparing low and high DMI, DM,

CP, NDF, and hemicellulose digestibility increased in sheep and cows with low intake

and high starch diets. Similar are the results of Pino and Heinrichs (2016b), where

although NDF, ADF, and hemicellulose digestibility were not affected, DM and starch

digestibility increased linearly as starch replaced dietary forage.

19

Impact of F:C

Precision feeding requires high energy, limit-fed diets; however, dairy farmers

preferentially feed dairy heifers with ad-libitum, low energy, forage-based diets.

Although limit-fed, high energy diets contain more concentrates, they are more cost

effective per unit of energy and protein than forages (Zanton and Heinrichs, 2007). High

concentrate diets have been shown to have greater efficiency of ME use (Blaxter and

Wainman, 1964). Also, high energy diets reduce DMI in dairy heifers to cover

requirements and reduce the nutrient loss that occurs in ad-libitum diets. Reduced DMI

and reduced loss of nutrients in feces and refusals benefit farm economy as it reduces the

cost of rearing. Also, the reduction in DMI reduces visceral organ size that reduces the

metabolic cost of digestion (Reynolds et al., 1991b). This allows more energy to be used

by the rest of the organs for growth (Huntington et al., 1996), and leads to an increase in

feed efficiency and a reduction in dairy heifer rearing cost (Zanton and Heinrichs,

2009b). Additionally, HC diets are more digestible due to the lower DMI and higher

rumen retention time, prolonging the interaction with microbes that degrade nutrients to a

larger extent (Wertz et al., 2001; Lascano and Heinrichs, 2009).

The concept of using higher concentrate proportions in ruminant diets has been

widely described before (Colucci et al., 1989; Reynolds et al., 1991b; Huntington et al.,

1996; Zanton and Heinrichs, 2008a; Lascano and Heinrichs, 2009; Suarez-Mena et al.,

2015; Lascano et al., 2016). Reynolds et al. (1991) and Huntington et al. (1996) observed

that at the same level of ME intake, steers that consumed HC diets had reduced heat

production and more energy was used for growth. Also, as diet concentrate increased,

nutrient digestibility was higher. Reynolds et al. (1991b) observed that with higher

20

concentrate in the diet, apparent digestion of DM, energy, CP, ether extract, ash, OM,

NDF, ADF, and hemicellulose increased in steers that consumed a low F:C (25:75).

Similar results were observed by Murphy et al. (1994) in lambs, when as diet concentrate

proportion increased (up to 92% of the ration) apparent digestibility of DM, OM, ADF,

NDF, and starch increased linearly. Colucci et al. (1989) found that apparent digestibility

of DM and energy increased at low and high DMI as dietary concentrate increased in cow

and sheep diets. Also, NDF digestibility increased linearly as dietary concentrate

increased, only at low DMI. In cows at low intakes, NDF, ADF, and hemicellulose

digestion increased linearly as dietary concentrate increased (Colucci et al., 1989). Thus

an interaction between F:C and DMI exists.

In dairy heifers apparent digestion for DM, OM, and ash increased with HC diets

(Zanton and Heinrichs, 2009a; Lascano and Heinrichs, 2011; Suarez-Mena et al., 2015;

Lascano et al., 2016; Zanton and Heinrichs, 2016). Lascano and Heinrichs (2011, 2016)

and Pino and Heinrichs (2016b) observed that apparent starch digestibility increased with

HC diets, and Lascano and Heinrichs (2011, 2016) observed that NDF and ADF

digestibility increased with HC diets. The other authors did not observe increments in

NDF, ADF, and hemicellulose digestion or found a reduction in digestion as in Zanton

and Heinrichs (2009a). This could be explained by a shift in rumen bacteria towards

amylolytic bacteria at expenses of fibrolytic bacteria, limiting fiber digestion (Mertens

and Loften, 1980; Calsamiglia et al., 2008).

Zanton and Heinrichs (2009a) observed higher CP and N digestibility in diets

with a low proportion of forages. Lascano et al. (2016) did not observe differences in N

digestion, but N excretion was reduced and N retention was higher in heifers fed HC

21

diets. Thus, researchers have observed that the reduction in excretion and the increase in

N retention leads to increased N efficiency and use by animals fed with HC diets

(Murphy et al., 1994; Moody et al., 2007; Zanton and Heinrichs, 2009b). In general, as

the F:C decreases, DM, OM, and starch digestion increases due to more energy available

in the rumen and rapid growth of microbes that can degrade nutrients faster than in diets

with high proportion of forages (Merchen et al., 1986). Also, in precision-fed dairy

heifers HC rations reduce DMI and stimulate rumen retention that will provide a higher

digestion response (Zanton and Heinrichs, 2009b).

Effect of NDF on digestion

Animal performance depends on intake and digestibility of nutrients. But, intake

and digestibility of nutrients are closely related to the amount and digestibility of dietary

NDF (Van Soest, 1967; Mertens, 2009). The physical properties and source of NDF are

important factors associated with ruminal degradation of nutrients. Forages and

concentrates have different NDF proportions, but due to physical and chemical

characteristics they also present differences in rumen digestion (Sarwar et al., 1991).

Diets with the same amount of NDF could have different nutrient digestibility depending

on the source of NDF (forages or concentrates; Mertens, 2009) or differences in the rate

and extent of NDF digestion (Varga and Hoover, 1983). It has been demonstrated that

NDF content and the variation in digestion are some of the most important factors

associated with changes in DM digestibility.

When low energy, high fiber rations are fed, ruminants regulate intake based on

rumen fill; however, when high energy, low fiber diets are fed, ruminants limit their

22

intake based on maintenance and production energy requirements (Colucci et al., 1982;

Van Soest, 1994; Mertens, 2009). Fiber content and its digestibility have a large impact

on nutrient digestion because fiber is the least digestible component of the diet. Intake is

regulated by the amount of rumen undigested NDF and also by rapidly digested NDF.

Cell walls that degrade rapidly in the rumen promote a greater rate of digestion and

nutrient rate of passage, leading to a greater DMI (Mertens and Loften, 1980; Varga and

Hoover, 1983; Oba and Allen, 2000; Mertens, 2009).

Oba and Allen (2000) observed that at higher DMI, rate of passage was higher

and NDF digestibiliy was lower. However, this does not occur in limit-fed heifers. As

DMI is limited, nutrients have a longer exposure time to rumen microbes, and

degradation and digestion are increased (Colucci et al., 1989; Zanton and Heinrichs,

2009b). Sarwar et al. (1990) studied the replacement of forage NDF with soyhulls or corn

gluten in dairy heifer ad-libitum diets and found that the diet with soyhulls or corn gluten

decreased rumen pH 3 h after feeding and decreased rumen NDF digestion when

compared to control (forage NDF). However, total tract NDF digestibility was higher and

OM digestibility was lower when feeding soyhulls or corn gluten compared to control.

The drop in OM digestion is because of lower rumen pH and its effect on fibrolytic

bacteria. Similar results were reported by Calsamiglia et al. (2008) using in vitro studies.

However, in precision-fed dairy heifers, as intake is reduced, pH does not affect

digestibility to a large extent. Minimum pH (reported in some precision feeding trials)

never fell below 5.5 and did not have effects on NDF digestibility (Lascano et al., 2009;

Pino and Heinrichs, 2016; Zanton and Heinrichs, 2016).

23

To demonstrate that nutrient digestibility increased with low intakes, Colucci et

al. (1989) evaluated low and high intake of HC diets in sheep and cows. For both species,

DM, energy, and CP digestibility increased in diets with low and high intake; however,

NDF, ADF, and hemicellulose digestibility only increased with low intake. Therefore,

low intake and HC diets increased fiber digestion due to higher fiber retention time of the

diet in the rumen. Also, NDF digestibility has been shown to increase in low intake

situations using low and high concentrete diets (Colucci et al., 1989). Ding et al. (2015)

observed that low quality forages (high in NDF) used in precision-fed heifers presented

lower DM and OM digestion than high quality forage diets. In precision feeding diets,

studies have demostrated that fiber digestibility does not decrease with HC, because there

is no drop in rumen pH and hence no reduction in fibrolytic bacteria (Pino and Heinrichs,

2016; Zanton and Heinrichs, 2016). Data available suggests that the amount of dietary

NDF does not directly affect nutrient or fiber digestibility. Even though fiber digestibility

depends of many factors (Mertens, 2009), more studies are necessary to compare

precision feeding diets to traditional ad-libitum diets to evaluate how these factors are

affected with low intake.

Effect of DMI and passage rate on nutrient digestibility

The nutritional value of ruminant diets is affected by the rate of nutrient

degradation and evacuation from the rumen. These 2 factors will determine the release of

nutrients by microbial fermentation and feed intake. Increasing feed intake usually

increases rate of liquid and particle passage through the rumen and the GI tract in

ruminants (Balch and Campling, 1965; Colucci et al., 1990). Higher amounts of dietary

24

fiber will increase the rate of passage principally by the increase in rumen load and

stimulation of evacuation (Clauss and Hummel, 2005). Bell (1971) determined that gut

capacity of herbivores remains a constant fraction of BW, and as BW increases there are

some specific tissue metabolic rates that decrease. In ad-libitum diets, the ratio between

organs and gut surface to digesta volume stays constant, but in precision feeding this ratio

increases due to changes in the GI tract volume. This allows a higher nutrient retention

time in the rumen and also greater surface of contact with gut enzymes for digestion and

absorption, and thus, greater digestibility (Clauss and Hummel, 2005).

As stated before, precision-fed heifers consume and digest fewer nutrients than

when fed traditional ad-libitum diets; reducing heat production due to digestive

metabolism and retaining more energy that can be used by tissues for growth (Reynolds

et al., 1991b). In ad-libitum fed heifers, passage rate is greater when high proportions of

concentrates are fed in the diet. This is explained by the smaller particle size of these

diets in comparison to forage-based diets (Van Soest, 1994) .With HC ad-libitum diets,

retention time is reduced, together with decreased rumen digestion of nutrients (Colucci

et al., 1982, 1990). However, in precision feeding heifers DMI is controlled to cover

energy and N requirements with energy dense diets, and then rumen retention time is

prolonged (Colucci et al., 1989; Murphy et al., 1994). With the reduction in intake, rate

of passage is reduced, increasing exposure time of feedstuffs to microbes, leading to an

improvement in nutrient degradation by rumen microbes; overall, nutrient digestibility is

increased in precision feeding dairy heifers (Colucci et al., 1990; Zanton and Heinrichs,

2008a; Zanton and Heinrichs, 2009b; Lascano et al., 2016).

25

Zanton and Heinrichs (2008a) offered 4 different levels of DMI (high forage

diets) and evaluated the rate of passage in dairy heifers. They observed that as DMI

increased up to ad-libitum levels, rumen passage rate also increased. Also the researchers

observed that DM, OM, and NDF digestibility increased as intake decreased, leading to

higher feed efficiency in limit-fed heifers. Lascano et al. (2016 ) showed that low forage

diets had lower turnover rate for solid and liquid fractions than higher fiber diets, and also

that rate of passage increased linearly as dietary fiber increased. These changes in

retention time allowed greater DM, OM, NDF, ADF, cellulose, and starch digestibility in

low forage diets, while DM, OM, and cellulose digestion decreased linearly as dietary

fiber increased. Retention time of rumen digesta is also affected by F:C. Lascano and

Heinrichs (2009) evaluated 3 F:C with 3 different levels of intake. Heifers that consumed

the smallest F:C presented a lower DM turnover rate leading to a higher rumen retention

time. Pino and Heinrichs (2016a) evaluated diets with 4 different starch concentrations

and 4 different DMI dairy heifers. As dietary starch concentration increased, DMI

decreased linearly. In this study, DM, hemicellulose, and starch digestibility increased

linearly as starch concentration increased; however, treatments did not affect NDF and

ADF digestion. Zanton and Heinrichs (2016) observed that heifers fed with low energy

diets and high DMI presented a greater ruminal passage rate and lower OM digestibility

at 8 and 20 mo of age. Also, they observed that heifers that received high energy diets

with lower DMI presented higher N digestion and retention when compared to low

energy, high intake diets. Colucci et al. (1989) observed that sheep and cows fed with low

intake diets presented greater digestion performance due to longer retention time in the

rumen.

26

Conclusions

New information and more focused research about precision feeding in dairy

heifers has been published in the last 10 years. Studies support that this feeding system

improves feed efficiency through a reduction in DMI, energy dense diets, and highly

digestible feedstuffs, covering the requirements of growing animals. By reducing DMI,

the metabolic expenses of the portal-drained viscera, liver, and kidneys was decreased by

a dramatic reduction in oxygen and glucose consumption. The reduction in DMI also

changes the passage rate of nutrients in the rumen, where rumen turnover is lower as

DMI decreases. Thus, feedstuffs stay in the rumen longer, and microbes have more time

to degrade nutrients, increasing diet digestibility. Also, precision feeding systems can

reduce the cost of rearing heifers. As less feedstuffs are used there are no refusals or

nutrient losses and there is a reduction in manure output.

The objective of this literature review was to analyze metabolic adaptations of

dairy heifers to precision feeding systems and to expose the effect of some nutrients on

precision feeding and digestibility. In the last decade seminal studies on precision feeding

have been performed; however, more research is needed to evaluate the impact of

specific nutrients in this system and compare digestibility and rumen fermentation in

precision feeding vs. ad-libitum diets.

27

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Zanton, G. I., and A. J. Heinrichs. 2016. Efficiency and rumen responses in younger and

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99:2825-2836.

Chapter 3

Effect of trace minerals and starch on digestibility and rumen fermentation in

diets for dairy heifers

A paper published in the Journal of Dairy Science1

F. Pino2 and A. J. Heinrichs*

3

A reprint is contained in the following pages.

1 Reprinted with permission of J. Dairy Sci., 2015. 99:2797–2810.

2 Primary researcher and author.

3 Author for correspondence.

*Department of Animal Science, The Pennsylvania State University, University Park, PA 16802

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Chapter 4

Sorghum forage in precision-fed dairy heifer diets

A paper published in the Journal of Dairy Science1

F. Pino2 and A. J. Heinrichs*

3

A reprint is contained in the following pages.

1 Reprinted with permission of J. Dairy Sci., 2017. 100:1–12.

2 Primary researcher and author.

3 Author for correspondence.

*Department of Animal Science, The Pennsylvania State University, University Park, PA 16802

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Chapter 5

Comparison of diet digestibility, rumen fermentation, rumen rate of passage,

and feed efficiency in dairy heifers fed ad-libitum versus precision rations

with low and high quality forages and 2 levels of neutral detergent fiber

Abstract

Reducing feed costs and improving feed efficiency are important considerations

when raising dairy heifers. To better understand these issues, this study compared ad-

libitum versus precision-fed diets with 2 forages and different levels of neutral detergent

fiber (NDF) to evaluate rumen fermentation, diet digestibility, feed efficiency, and

digesta passage rate. Eight Holstein heifers (18.4 ± 0.6 mo and 457.2 ± 27.29 kg body

weight) fitted with rumen cannulas were used in a 2-factor split-plot, Latin square design

with 19-d periods (14 d of adaptation and 5 d of sampling). The whole-plot factor was

feeding system with ad-libitum or precision feeding and 4 heifers in each plot. The

subplot included 2 factors: forage quality (low quality: grass hay, LFQ; high quality:

corn silage, HFQ) and NDF content (high NDF, 48 % HNDF; low NDF, 39.8 %,

LNDF). Diets were formulated to provide the same energy level (0.234 Mcal of

metabolizable energy intake/kg of empty body weight0.75

) for precision-fed and 110% of

previous intake for ad-libitum-fed heifers; all diets were balanced to contained 12.75%

crude protein. Total collection of urine and feces was completed for 4 d to determine

digestibility of nutrients. Rumen contents were evacuated pre and post feeding to

measure mass and volume and to estimate rate of passage, rate of digestion, and rumen

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turnover time. Precision-fed heifers showed enhanced feed efficiency over ad-libitum

heifers. Forage quality and NDF level affected dry matter intake, increasing with HFQ

and decreasing with HNDF. Thus, the best feed efficiency was observed in HFQ-LNDF-

precision feeding diets. Mean rumen pH was lower for ad-libitum than for precision

feeding diets, but HNDF diets acted as a buffer, regulating rumen pH. Volatile fatty acid

concentrations were affected principally by forage quality. Urine excretion was affected

by the type of diet, and precision feeding diets produced more urine. Rate of passage, rate

of digestion, and turnover time were affected principally by the type of diet, where ad-

libitum diets showed faster rate of passage for solid feeds and fluids, increased rate of

digestion, and shorter retention time in the rumen. However, both NDF level and forage

quality also modified rumen passage rate and retention time. Feed efficiency was

improved in precision-fed heifers, representing an important opportunity for reducing the

cost of feeding dairy heifers.

Key Words: dairy heifer; precision feeding

Introduction

Reducing costs associated with raising heifers is one of the important topics in

present day dairy farming. Heifers are conventionally fed with high amounts of low

quality forages in an attempt to reduce feed costs. However, it is clear that improving

feed efficiency in heifers is one of the best ways to improve growth performance and

reduce feed costs (Lascano and Heinrichs, 2009; Pino and Heinrichs, 2016; Zanton and

Heinrichs, 2009b). Feed efficiency can be improved by reducing DMI and increasing

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nutrient density (Hoffman et al., 2007; Zanton and Heinrichs, 2009b). Diets that reduce

DMI, condense energy, and use highly digestible feedstuffs are often referred to as

precision feeding diets. Reducing DMI in growing heifers increases nutrient efficiency

due to a reduction in the metabolic expenses of nutrient absorption and oxidative

metabolism for maintenance (Reynolds et al., 1991), thus more energy will be available

to be used in growth (Lascano and Heinrichs, 2011; Moody et al., 2007; Zanton and

Heinrichs, 2009b). Precision diets cover the requirements to provide enough energy and

nutrients for adequate, economical growth without affecting future performance and milk

production (Zanton and Heinrichs, 2009b).

In precision-fed diets, high amounts of concentrate used to produce an

energy dense diet do not have dramatic impacts on rumen pH due to the limited amounts

of DM fed, and rumen fermentation is therefore positively affected by having greater

fiber digestion and larger amounts of rumen bacterial populations (Lascano and

Heinrichs, 2009; Moody et al., 2007; Pino and Heinrichs, 2016). While it is true that

precision diets improve feed efficiency, there is a limited amount of data comparing ad-

libitum with precision diets for heifers. Moody et al. (2007) and Lascano et al., (2009)

demonstrated that with low forage-to-concentrate ratios (F:C) heifers were more efficient

using corn silage as a forage source. These studies also included some evaluation of the

effect of NDF on digestibility and efficiency. Zanton and Heinrichs (2009a) determined

that N retention and efficiency decreased as DMI increased and also that heifers had

improved N retention with limit-fed diets.

Limited studies have evaluated rate of passage in precision-fed dairy

heifers, however it is reported that as forage increases in the diet, retention time in the

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rumen is greater and rate of passage is reduced (Colucci et al., 1990; Lascano and

Heinrichs, 2009). A lower passage rate increases retention time in the rumen, with

microbial growth and feedstuff degradation increasing, leading to greater digestion by the

animals (Colucci et al., 1990; Mertens and Loften, 1980). Also, in cows the effect of

NDF on intake and rate of passage has been demonstrated such that higher intake and

lower NDF in the rumen stimulate the turnover rate (Dado and Allen, 1995). Mertens

(2009) and Oba and Allen (2000) also showed that low NDF diets create higher rates of

passage, however this information is from lactating cows and has not been shown in

precision-fed dairy heifers.

In general there are many factors that could affect digestibility including forage

quality, NDF level in the diet, and overall diet digestibility. Previous studies from our lab

have observed that changes in DMI, NDF, and concentrate in precision-fed diets can

modify rumen fermentation and total tract nutrient digestibility (Ding et al., 2015; Moody

et al., 2007; Pino and Heinrichs, 2016). However, limited information is available

comparing traditional ad-libitum feeding to precision diets. Also, it is possible

interactions among the factors previously mentioned could affect feed efficiency in

precision-fed diets by modifying ruminal fermentation or digestibility. Therefore, the

objective of this study was to evaluate effects and possible interactions of DMI, forage

quality, and diet NDF level on feed efficiency, rumen fermentation, and rate of passage.

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Materials and Methods

Animals, Treatments, and Experimental Design

All procedures involving the use of animals were approved by the Pennsylvania

State University Institutional Animal Care and Use Committee (#46266). Eight Holstein

heifers (18.4 ± 0.6 mo and 457.2 ± 27.29 kg BW) fitted with a 10-cm silicone rumen

cannula (Kehl, SP, Brazil) were used in a 2-factor split-plot, Latin square design with 19-

d periods (14 d of adaptation and 5 d of sampling). The whole-plot factor was feeding

type with ad-libitum or precision feeding and 4 heifers in each plot. The subplot included

2 factors: forage quality and NDF content.

Heifers were kept in tie-stalls 10 d before the experiment began to adapt them to

the facility and management and then were randomly assigned to treatments. Heifers

were weighed weekly, and BW was determined by the average of 2 measurements taken

on the same day. The amount of TMR offered during the experiment was adjusted

weekly based on BW to allow an average of 1.0 to 1.1 kg/d of ADG. Heifers were housed

in individual tie-stalls in a mechanically ventilated barn with free access to water in the

stalls. The animals were released to a paved exercise pen for 3.5 h/d on non-sampling

days.

Diets

Rations were mixed in a Calan Super Data Ranger (American Calan, Northwood,

NH) for the 5 d of sampling. Grain mixes were prepared before each period as a single

mix, and corn silage DM was measured before ration mixing in a microwave as described

by Pino and Heinrichs, (2014). Water was added to the grass hay diets to provide the

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same moisture as the corn silage diets and to help with the mixing process. The different

diets were kept in closed nylon bags to avoid oxygenation and stored in a cooler at 6 ◦ C

until fed.

Rations were formulated to provide low or high quality forages (grass or corn

silage respectively) and low or high values of NDF (39.8 or 48.0% NDF on DM basis).

Rations provided a 60:40 F:C. Predicted DMI was calculated based on energy intake for

precision-fed diets, grain mixes were formulated to provide the same energy level (0.234

Mcal of ME intake/kg of empty BW0.75

), and all diets balanced to contained 12.5% CP. In

the ad-libitum diets energy intake was determined based on the requirements in NRC

(2001) for heifers gaining 1 kg/d. Ad-libitum heifers were fed at 110% of expected

intake, and rations were fed daily as TMR at noon. Eating time (h to consume the whole

ration) was recorded daily during sampling days, and orts were weighed, dried, and saved

for further analysis.

Sample Collection and Analysis

Feedstuffs were collected before every period, and TMR was collected during the

mixing day to be dried in a forced-air oven at 55°C for 48 h to measure DM and ground

through a 1-mm screen (Wiley mill, Arthur H. Thomas, Philadelphia, PA) for further

analysis. Particle size was analyzed (Penn State Particle Separator with 19-, 8-, and 4-mm

sieves) during the mixing day using a composite of the 4 d per treatment.

Urine was collected from d 15 to 19 using the modified cup collector (Lascano et al.,

2010) with the objective to avoid fecal contamination. During sampling days total urine

was weighed and recorded daily after feeding. In addition, feces were collected and

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stored in airtight containers. After feeding, daily feces were mixed, and a subsample was

saved at 4°C to be composited at the end of each period. Then the subsample was dried in

a forced-air oven at 55°C for 72 h and ground through a 1-mm screen (Wiley mill, Arthur

H. Thomas, Philadelphia, PA) until analysis.

The composited and dried feeds, fecal, and orts samples were analyzed for DM, ash,

and CP, (AOAC, 2000 methods 934.01, 942.05 and 968.06 respectively); NDF and ADF

were analyzed individually (Van Soest et al., 1991). Analysis of NDF included use of

heat-stable α-amylase (Sigma Chemical Co., St. Louis, MO) and sodium sulfite (Van

Soest et al., 1991), using an Ankom200

fiber analyzer (Ankom Technology Corp.,

Fairport, NY). Crude protein was calculated from feeds that were analyzed by

Cumberland Valley Analytical Services, Inc. (Maugansville, MD). Starch was

determined by the modified method of (Hall, 2008) using previously ground samples and

Hazyme enzyme (Centerchem, Norwalk, CT). Metabolizable energy intake was estimated

for each heifer within each period using the observed OM intake × 4.409 × 0.82 as

described in NRC (2001).

Rumen contents were evacuated manually through the cannula at 10 a.m. (2 h before

feeding) on d 14 and at 3 p.m. (3 h after feeding) on d 19 for ad-libitum diets and 4 p.m.

(4 h after feeding) for precision-fed heifers. The time between feeding and rumen

evacuation was different for ad-libitum and precision diets to ensure we evacuated the

rumen near the time of maximum capacity. Ad-libitum-fed heifers stopped eating 2 to 3 h

after feeding, and precision-fed heifers consumed the whole meal between 3 and 4 h after

feeding. After the evacuation on d 19, rumen digesta was switched between heifers to

assist in adaptation for the next period. Mass and volume were determined for ruminal

74

content, and a 500 mL sub-sample was saved to determine pool size of nutrients in the

rumen of each diet to then calculate ruminal passage rate. Ruminal pool sizes were

calculated by multiplying the digesta DM weight by the concentration of each

component. Rumen kinetics were calculated using indigestible NDF (iNDF) and

potentially digestible NDF (pdNDF) as reported in (Dado and Allen, 1995; Oliver et al.,

2004). Fractional passage rate (Kp) or turnover rate in the rumen was calculated as %/h =

(intake of component / ruminal pool of component) / 24 × 100 and fractional digestion

rate (Kd) as %/h = ((pdNDF intake / 24) / pdNDF in pool size) - Kp.

Chromium EDTA solution (equivalent of 2.5 g of Cr/heifer; (Uden et al., 1980) was

used as a ruminal liquid passage rate marker. Rumen contents (3 kg) were evacuated,

mixed with chromium EDTA solution, and returned to the rumen at time 0 on d 18.

Rumen samples were taken on d 18 from 5 locations in the rumen (dorsal, ventral,

anterior, caudal, and central) at 0, 0.5, 1, 2, 4, 6, 8, 12, 16, 20, 22, and 24 h relative to

feeding time. Rumen fluid was mixed and strained through a 0.28-mm fiberglass mesh

screen (New York Wire, Mt. Wolf, PA), pH was recorded (pH meter, model M90,

Corning Inc., Corning, NY), and 5 mL of strained fluid saved with 1 mL 0.6% 2-

ethylbutyric and 1 mL 25% metaphosphoric acid at -20°C for VFA analysis (Yang and

Varga, 1989). Another 50 mL were saved at -20°C and then sent to SDK laboratories,

Inc. (Hutchinson, KS) for Cr quantification using microwave acid digestion and an

atomic absorption spectrophotometer.

Chromium concentrations were used to determine Cr dilution rates from the rumen as

described in (Bartocci et al., 1997). Liquid passage rate (Kl) was calculated as the slope

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of the semilog plot of Cr concentration against time. The equation used to describe the

disappearance curve was:

Y= ae-Kl t

Where: Y= marker concentration at t time; a= marker concentration at zero time;

Kl = dilution constant of marker. Rumen fluid volume (L) was estimated dividing the

amount of Cr by the antilog of the intercept (a) at time zero. The flow rate (L/h) was

calculated by multiplying the volume of ruminal fluid by the outflow rate (Kl).

For determination of in situ digestion at 24, 30, and 48 h, feedstuffs and diets were

mixed in the laboratory according to the diet used during sampling days. Samples were

ground through a 1-mm screen and 5 g weighed in triplicate into ANKOM bags (pore

size 50 ± 15 µm), closed with zip ties in 2 sites. Then, samples were placed in a laundry

bag and were tied with a cord to the cannulas. Bags were placed in warm distilled water

for 15 min before being inserted into the rumen and were incubated for 48, 30, and 24 h

relative to feeding time to calculate rumen digestion at each time point. Bags were

removed at feeding time, separated, rinsed manually with tap water, and then washed in a

washing machine for 2 min 3 times (rinse cycle). Bags were rolled and dried in a forced

air oven at 55°C for 72 h. Then samples were weighed to determine DM digestibility, and

a subsample was analyzed to evaluate NDF digestibility. The proportions of iNDF in the

pool samples were determined incubating the diets for 12 d in the rumen. Potential extent

of NDF digestion (PED = 100 × pdNDF / (pdNDF + iNDF)) was calculated (Grant,

1994a; Grant, 1994b), where pdNDF is the potentially digestible NDF as proportion of

the initial DM (NDF - iNDF).

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Statistical Analysis

All statistical analyses were conducted in SAS (Version 9.4, SAS Institute Inc.,

Cary, NC) using the MIXED procedure. Dependent variables were analyzed as a 2-factor

split-plot, Latin square design with the diet (ad-libitum or precision feeding) as the whole

plot and forge quality and NDF content as the subplot factors. Heifers were considered

experimental units because they were individualy fed and intake and ADG were known.

All denominator degrees of freedom for F-tests were calculated according to Kenward

and Roger (1997).

The model used was:

Yijkl= µ + Ti+ Fj+ Lm + Nk(i) + TF(ij) + TL(im) + FL(jm) + TFL(ijm) + Pl + eijkml

where Yijkl is a continuous dependent response variable; µ is the overall mean; Ti

is the fixed effect of diet treatment (i = 1,2); Fj is the fixed effect of forage quality (j =

1,2); Lm is the fixed effect of NDF level (m = 1,2); Nk(i) is the random effect of heifer

within the diet treatment; TF(ij) is the interaction of diet and forage quality; TL(im) is the

interaction of diet and NDF level; FL(jm) is the interaction of forage quality and NDF

level; TFL(ijm) is the three-way interaction between type of diet, forage quality and NDF

level; Pl is the period effect and eijkml is the residual error. TFL(ijm) was not significant and

was removed from the final model. Repeated measures were used to analyze rumen pH

and VFA using the SP(POW) covariance structure for time intervals not evenly spaced.

Time and time by treatment interaction were included in the model for rumen pH and

VFA. Interaction with time are stated in the text if significant. Residual variances were

assumed normally distributed, and all data is presented as LSM. P-values for treatments

and interactions will be presented in tables. Residuals over ± 3 SD were considered

77

outliers and were removed prior to analysis. Differences were declared significant at P ≤

0.05 and tendencies at P ≤ 0.10 for main effects.

Results and Discussion

Ingredients, chemical composition, and particle size of the diets are presented in

Table 1. The proportions of ingredients between ad-libitum and precision diets were not

equal by design, because the objective was to make precision feeding diets more

digestible and reduce DMI. Canola meal was used to balance CP content for all the

treatments (12.8% CP). Neutral detergent fiber was formulated with 2 levels for ad-

libitum and precision feeding (39.8 or 48.0% NDF). Also, ADF was higher in diets that

contained higher concentrations of NDF. The hemicellulose content was higher in the

diets with low quality forages due to higher content of hemicellulose in grass hay than

corn silage. Starch content was different for all diets, but was higher in diets that had high

forage quality (HFQ) due to the proportion of starch and low NDF (LNDF) level in the

corn silage. Starch was lower in diets with low forage quality (LFQ) and high NDF

(HNDF). Ad-libitum diets were formulated to provide enough ME to gain 1 kg/d

according to NRC (2001). On the other hand, precision-fed diets were formulated to

provide 0.21 Mcal ME/kg of empty BW0.75

, which allowed for an ADG close to 1 kg/d

(Zanton and Heinrichs, 2009a). Physically effective fiber was variable between diets, but

each contained enough for adequate rumination (> 25% for all diets).

Body weight, intakes, and feed efficiency are presented in Table 2. Heifers fed ad-

libitum diets consumed 2 kg more DM than precision-fed heifers (P ≤ 0.01).

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Furthermore, heifers fed ad-libitum diets consumed 1.86 kg more DM when the diet

contained HFQ than LFQ (P ≤ 0.01) and only 0.43 kg more DM in the case of HNDF (P

≤ 0.01). Precision-fed diets resulted in greater feed efficiency than ad-libitum diets (feed

to gain ratio of 8.59 vs. 10.45; P ≤ 0.01) due to the increased DMI in ad-libitum diets and

similar ADG between the types of diet. Feed efficiency was also greater for heifers that

consumed LNDF compared to HNDF (P ≤ 0.01). Furthermore, an interaction between

diet and forage quality affected feed efficiency (P = 0.03). Greater feed efficiency is one

of the principal objectives resulting from precision feeding programs (Hoffman et al.,

2007; Zanton and Heinrichs, 2008) that makes it an effective tool to reduce heifer raising

costs (Zanton and Heinrichs, 2009b).

Heifers fed ad-libitum diets, HFQ, and HNDF had greater intakes of NDF, iNDF,

pdNDF, and ADF when compared to heifers fed precision diets, LFQ, and LNDF,

respectively. Furthermore, NDF intake had a tendency (P = 0.08) to be affected by the

interaction between type of diet and forage quality. In addition, interactions between

forage quality and NDF level affected iNDF and ADF intakes (P = 0.04 and P = 0.01,

respectively). Hemicellulose intake was increased through feeding ad-libitum diets (P =

0.02) and HNDF levels (P = 0.02).

Starch intake was increased in ad-libitum diets (P = 0.02), HFQ (P ≤ 0.01), and

LNDF (P ≤ 0.01) when compared to heifers fed precision diets, LFQ, and LNDF,

respectively. Crude protein intake was higher for LFQ than HFQ (P = 0.03). There was

also an interaction between type of diet and NDF level (P = 0.02). These are a result of

the higher concentration of CP in grass hay. The higher DMI in the ad-libitum diets led to

increased CP intake in these diets that also contained HNDF levels. Metabolizable energy

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intake was only affected by FQ, with LFQ diets containing more ME than the HFQ (P =

0.01).

While precision-fed diets often lead to improved feed efficiency and reduced DMI

(Lascano and Heinrichs, 2011; Moody et al., 2007; Pino and Heinrichs, 2016; Zanton and

Heinrichs, 2008), this is the first study directly comparing precision-fed with ad-libitum

diets. The type of forage and amount of fiber also modifies and regulates heifer DMI

(Lascano and Heinrichs, 2011; Zanton and Heinrichs, 2008), indicating that the best way

to improve feed efficiency is to utilize a precision feeding program with high quality

forages and low NDF diets.

Rumen pH, eating time, and VFA are presented in Table 3 and Figures 1 and 2. Mean

pH was lower for ad-libitum vs. precision-fed diets (6.37 vs. 6.59; P ≤ 0.01) as well as

LNDF vs. HNDF (P = 0.04). In general, rumen pH for ad-libitum-fed heifers was lower

but more homogeneous throughout the day (Figure 1) than for the precision-fed heifers,

reflecting both increased intakes and a more even eating pattern throughout the day.

Maximum pH was only affected by type of diet, where precision-fed diets reached a

higher pH than ad-libitum diets (7.28 vs. 6.83; P ≤ 0.01). On the other hand, minimum

pH was lower for precision-fed diets and HFQ (P = 0.05 and P ≤ 0.01 respectively) and

showed a tendency to be lower for LNDF (P = 0.06). An interaction was observed

between type of diet and forage quality, where precision feeding diets with HFQ had the

lowest minimum pH (P ≤ 0.01). This is likely due to precision-fed heifers consuming

their ration rapidly, resulting in greater amounts of fermentation at one time and a drop in

pH (Table 3; Figure 1). Heifers fed diets with HFQ had lower rumen pH than LFQ (P ≤

0.01), likely due to higher starch levels in corn silage compared to grass hay.

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The time that heifers spent consuming the meal was lower for precision-fed heifers

(4.2 vs. 24 h after feeding; P ≤ 0.01), due to precision-fed heifers consuming less DMI.

This also resulted in precision-fed heifers having a greater eating rate (2.66 kg/h vs. 0.481

kg/h; P ≤ 0.01). The time spent consuming rations by precision-fed heifers was similar to

previous studies (Pino and Heinrichs, 2016).

Total VFA concentrations were increased for ad-libitum diets (P = 0.05; Table 3), as

would be expected from higher DMI. Production of VFA peaked for precision-fed diets

between 4 and 6 h after feeding (Figure 1), similar to previous observations (Lascano and

Heinrichs, 2009; Pino and Heinrichs, 2016). In general, ad-libitum diets did not show a

peak, and the total VFA concentration was homogeneous throughout the day with the

exception of LFQ-LNDF that showed a peak 6 h after feeding (Figure 1).

The type of diet did not affect acetate proportion. However, LFQ diets containing

grass hay had higher acetate proportions compared with HFQ diets with corn silage (P ≤

0.01), as did rations with HNDF compared with LNDF (P ≤ 0.01). There was an

interaction between type of diet and forage quality (P ≤ 0.01), where the precision-fed-

HFQ had a lower acetate proportion than ad-libitum-HFQ. In addition, we can deduce

that the low pH showed by the HFQ diets in the precision-fed heifers 4 to 8 h after

feeding reduced rumen acetate proportion for those diets (Figure 2), likely due to changes

in fiber digestion during this time (Calsamiglia et al., 2008). Also, there was a tendency

for an interaction between forage quality and NDF level that affected rumen acetate

levels (P = 0.10).

Propionate was increased in diets containing HFQ (P ≤ 0.01), likely a result of HFQ

diets containing corn silage and LFQ diets containing grass hay. Also, the type of diet

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and NDF level interacted (P ≤ 0.01). There was an early spike in propionate proportion

for precision-fed diets containing corn silage (Figure 2), and ad-libitum diets containing

corn silage had higher proportions of propionate throughout the day. Propionate was

higher from 2 to 20 h after feeding for HFQ-HNDF in precision diets (P ≤ 0.01) and from

only 1 to 3 h in the HFQ-LNDF (P = 0.01). In this study the proportion of propionate

presented in LFQ was similar to the results of (Lascano and Heinrichs, 2009) using corn

silage as a forage source.

Diets with LNDF had higher butyrate levels than HNDF diets (P ≤ 0.01), and there

were interactions between NDF level and forage quality as well as NDF level and type of

diet. Type of diet and forage quality interacted as well (P ≤ 0.01); ad-libitum LFQ diets

had increased butyrate compared to ad-libitum HFQ diets, and the opposite was true of

precision diets with precision HFQ resulting in greater butyrate than precision LFQ diets.

Butyrate levels increased substantially in precision-fed HFQ-LNDF diets. The high

proportion of wheat middlings in this diet would result in fast fermentation, which could

explain the spike in butyrate. This also likely explains the observed drop in pH. With the

exception of precision feeding HFQ-LNDF, the other diets had similar values to previous

studies with the same F:C and alfalfa as a forage source (Pino and Heinrichs, 2016).

Acetate-to-propionate ratio (A:P) of LFQ diets was higher than HFQ diets due to the

higher acetate proportion in the rumen from digestion of grass hay instead of corn silage.

In addition, there was an interaction between type of diet and NDF level (P = 0.02). Also,

type of diet and forage quality by time interactions were detected for total VFA, acetate,

butyrate, and propionate (P ≤ 0.01 for all), suggesting that the timing of fermentation of

nutrients was different and the rate of digestion of the different diets was affected over

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time. There was no interaction between NDF level and time, suggesting that the different

amounts of NDF did not affect the rate of NDF digestion.

Overall, rumen fermentation was affected by type of diet, forage quality, and NDF

level. The effect of VFA on pH over time is clear and explains most of the changes in pH

in the rumen. This information is in agreement with (Calsamiglia et al., 2008) who stated

that changes in fermentation are explained by changes in microbial populations. In

general, we observed that ad-libitum diets presented a homogeneous pattern of

fermentation with lower variation in pH. However, precision feeding diets provided an

adequate rumen environment with no indication that fibrolytic bacteria or fiber

digestibility were negatively affected (Lascano and Heinrichs, 2009; Moody et al., 2007;

Pino and Heinrichs, 2016; Zanton and Heinrichs, 2008).

Excretion parameters are presented in Table 4. Wet feces production was not affected

by treatments, but dry feces weights were higher with HNDF diets compared to LNDF

diets (P = 0.02). Hoffman et al. (2007) reported a decrease in dry feces for heifers

consuming reduced DMI. However, the current study showed only a tendency (P = 0.10)

for precision-fed heifers to excrete less dry feces. Urine was increased for precision

feeding diets. It has been observed in our previous experiments that precision-fed dairy

heifers in a confined environment may consume more water, leading to increased urine

production. This could be a result of confinement or lower DMI (DeVries and von

Keyserlingk, 2009; Greter et al., 2011); however, higher urine production resulted in

increased total manure excretion for precision-fed heifers.

Apparent total tract nutrient digestibility and in situ digestibility are presented in

Table 5. Starch digestibility was the only parameter evaluated that was affected by the

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type of diet. Ad-libitum diets had higher starch digestibility than precision-fed diets (P =

0.01). These results were opposite of what we would expect due to the higher Kp for the

ad-libitum diets. Furthermore, there are no indications that greater microbial synthesis

(due to higher DMI and more stable pH throughout the day) increased digestion, because

the other nutrients were not affected. Corn silage diets had increased starch digestion

compared to grass hay diets (P ≤ 0.01). While these values were statistically different,

biologically it is of little importance because starch digestion rates were all above 98%.

Dry matter digestibility was affected by forage quality and NDF level. Diets with

corn silage were, on average, 5.1% more digestible than diets containing grass hay (P ≤

0.01). Also, LNDF diets were on average 4.28% more digestible than HNDF diets.

Digestibility of NDF was not affected by any treatment. However, ADF digestibility was

increased in HFQ diets (P = 0.03), likely due to ADF intake being slightly higher in these

diets. Hemicellulose digestibility was decreased in diets formulated with HFQ (P = 0.05)

as hemicellulose intake was higher in diets containing LFQ.

The type of diet did not affect in situ DMD; however, in situ DMD was increased in

diets containing LFQ and LNDF. Also, we observed an effect from an interaction

between forage quality and NDF level at 24 and 30 h after feeding (P ≤ 0.01 and P = 0.03

respectively). At all measurement times the HFQ-HNDF diet had lower DMD, likely due

to higher iNDF (Tables 6 and 7) and higher NDF intake for this diet.

In general, apparent total tract digestibility of nutrients was not affected by type of

diet. We expected greater digestibility for the precision diets due to the lower Kp

observed and reports from Zanton and Heinrichs (2009b) and Reynolds et al. (1991)

stating digestibility of feedstuffs was dependent on DMI. However, when DMI is

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reduced, it is necessary to increase energy to maintain rumen bacteria populations and

keep the same digestion rates. In this study, diets for precision feeding were formulated

with lower ME than ad-libitum diets which could partially explain why we did not

observe higher digestibility.

Pre-feeding rumen digestion kinetics are presented in Table 6 with sample collection

at 22 h after feeding. Rumen volume and mass were increased for HNDF diets (P ≤ 0.01

and P = 0.05 respectively) at this point. Also, there were interactions between diet and

forage quality for rumen volume and mass (P ≤ 0.01). Precision diet-LFQ had greater

volume and mass than ad-libitum diet-LFQ while ad-libitum diet-HFQ presented greater

volume and mass than precision diet-HFQ. In addition, an interaction between forage

quality and NDF level affected rumen mass (P = 0.05) and had a tendency to affect

rumen volume (P = 0.08). Rumen content density was higher for precision-fed heifers (P

= 0.04) and tended to be higher for HFQ diets (P = 0.10). Also, an interaction between

type of diet and NDF level tended to modify the rumen density (P = 0.08). Values were

lower than those shown in adult cows (Dado and Allen, 1995).

Rumen pool DM (%) showed a tendency (P = 0.08) to be affected by the type of diet,

where the precision-fed digesta contained more moisture than the ad-libitum digesta. This

result suggests increased water intake, which explains the increased urine production

presented in Table 4 for precision-fed heifers. Also, rumen pool DM (%) was increased

for HFQ and HNDF diets (P ≤ 0.01 for both), suggesting that grass hay diets with low

NDF may stimulate water intake. An interaction was observed between forage quality

and NDF level (P = 0.02), where HFQ-HNDF diets contained the least moisture. In the

same way, pool mass was increased in diets containing HFQ and HNDF (P ≤ 0.01 for

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both). Furthermore, there was an interaction between these 2 treatments (P ≤ 0.01) where

the heaviest pool mass was for the HFQ-HNDF diet. In agreement with other researchers

(Lascano and Heinrichs, 2009; Moody et al., 2007), we observed that NDF has a very

important role in rumen mass and volume in heifers. Diets with HNDF were heavier than

LNDF in wet and dry state, suggesting that diets with increased NDF will be retained in

the rumen longer.

Type of forage and NDF level affected the proportion and amount of NDF in the

pool. Corn silage diets had a higher NDF proportion and weight in the pool than grass

hay diets (P ≤ 0.01). This could be a result of corn silage diets requiring more time to

digest NDF than grass hay diets or increased NDF intake for corn silage diets. The

highest NDF proportion and weight was shown in the HFQ-HNDF diets (P ≤ 0.01).

Interestingly, iNDF proportion was also affected by forage quality but in an opposite

way. In this case, grass hay diets had higher proportions of iNDF than corn silage diets (P

≤ 0.01), but due to the total amount of pool being higher in the corn silage diets, the total

amount of iNDF was not affected by forage quality (P = 0.47). The iNDF proportion

showed a tendency to be affected by NDF level (P = 0.06), but HNDF diets had increased

total amounts of iNDF (P ≤ 0.01). An interaction between forage quality and NDF level

showed a tendency (P = 0.08) to affect the total amount of iNDF and was higher for the

HFQ-HNDF diet.

Potentially digestible NDF (pdNDF) in the pool was increased for HFQ and HNDF

levels (P ≤ 0.01 and P = 0.02 respectively). Also, forage quality and NDF level

interaction showed a tendency to affect the pdNDF content in the pool (P = 0.06). In

addition, digestible fraction was increased for LNDF diets (P ≤ 0.01). The proportion of

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NDF digested in the rumen was higher for the grass hay-based diets (P = 0.03) than corn

silage diets, and LNDF diets tended to have a higher proportion of NDF digested in the

rumen than HNDF diets (P = 0.09).

When analyzing turnover rate it was noted that even though ad-libitum diets had

numerically higher Kp for all nutrients analyzed, only iNDF Kp showed a tendency to be

increased (P = 0.08). Dry matter Kp was higher for LFQ diets and LNDF (P = 0.05 and P

= 0.03 respectively). An interaction was observed between forage quality and NDF level

(P ≤ 0.01), where grass hay diets with HNDF showed a higher Kp of DM, but for the

corn silage diets the effect was opposite with LNDF presenting higher DM Kp. In the

case of NDF, Kp was greater for LFQ. Similar to DM Kp, NDF Kp was effected by an

interaction between forage quality and NDF level (P = 0.01) where Kp for HNDF level

was higher in grass hay diets but lower for HNDF level in corn silage diets. The turnover

rate for pdNDF was not affected by treatments and only showed a tendency to be affected

by the interaction between forage quality and NDF level (P = 0.06). Passage rate of iNDF

was increased for HNDF (P = 0.01.)

Turnover time for DM was affected by forage quality and NDF level (P = 0.01 and P

≤ 0.01 respectively). Corn silage diets with HNDF exhibited higher turnover times than

corn silage diets with LNDF, while grass hay diets exhibited the opposite results. Neutral

detergent fiber turnover was affected only by forage quality, where HFQ diets were kept

longer in the rumen at this time point (P ≤ 0.01). The same interaction observed for DM

turnover time was also observed for NDF turnover, where grass hay diets with LNDF

resulted in longer turnover time and for corn silage diets the opposite was true. Turnover

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of iNDF was modified only by the NDF level with LNDF diets having longer turnover

times (P = 0.01).

Post-feeding rumen kinetics are presented in Table 7; results were different from

those of pre-feeding. Rumen volume and mass were greater for precision-fed heifers (P =

0.05, 0.04 respectively) and LFQ (P = 0.01 for both). Furthermore, at this time (4 h after

feeding) the precision-fed heifers had consumed their entire ration while ad- libitum

heifers consumed their ration more slowly and thus had consumed less of their total

ration. The increased volume and mass for LFQ is probably explained by higher water

consumption because the DM mass was not affected by forage quality.

Neutral detergent fiber pool proportion was increased in diets containing HFQ and

HNDF (P ≤ 0.01 for both). An interaction between type of diet and forage quality showed

a tendency (P = 0.09) to affect the NDF proportion in the pool, where the ad-libitum HFQ

diets showed the highest NDF pool proportion. The amount of NDF in the pool was

higher for HNDF diets (P = 0.04), which is to be expected. There was a tendency (P =

0.08) for precision-fed diets to result in a greater amount of NDF in the pool. This

tendency can be explained due to the precision-fed heifers consuming the whole ration by

the time we did the rumen evacuation. There was also an interaction between diet and

forage quality (P = 0.05).

Indigestible NDF as proportion of the pool was increased for diets with LFQ and

HNDF (P = 0.01 and P ≤ 0.01 respectively). Also, the total amount of iNDF in the pool

was affected by the type of diet and forage quality, while also showing a tendency to be

modified by the NDF level. Precision-fed diets showed higher amounts of iNDF than ad-

libitum diets (P = 0.04), due to the whole meal being in the rumen at the time of the

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rumen evacuation. The LFQ diets presented higher iNDF in the pool than HFQ diets (P =

0.03) even though iNDF intake was higher in the HFQ diets. This is due to iNDF in grass

hay diets remaining longer in the rumen. Grass hay diets had a higher iNDF retention

time in the rumen (P ≤ 0.01; Table 7). The HNDF diets tended to have higher amounts of

iNDF in the pool post feeding (P = 0.08).

Post-feeding pdNDF values were much higher than pre-feeding, showed a tendency

to be affected by forage quality (P = 0.10), and were increased for HNDF level (P =

0.04). This was due to the greater amount of NDF in the pool for diets containing HNDF.

Precision-fed-LFQ diets contained more pdNDF than precision-fed-HFQ diets, but for

ad-libitum diets the opposite was true (P = 0.02). The digestible fraction was greater for

LNDF diets. These results are in agreement with Grant (1994b) and Mertens (2009), who

stated that NDF is the part of forages that determines the rumen digestible fraction.

However, NDF rumenal digestion was not only affected by NDF level. It was also

affected by forage quality. Ruminal digestion of NDF was higher for grass hay diets (P ≤

0.01) and LNDF diets (P = 0.05).

Ad-libitum diets had greater turnover rates for all of the nutrients evaluated (P ≤ 0.01

for all). In this study, DM Kp was 7.1 vs. 4.9 %/h; NDF Kp was 3.1 vs. 2.1%/h; pdNDF

Kp was 3.5 vs. 2.5%/h and iNDF Kp was 2.4 vs. 1.6%/h for ad-libitum vs. precision

feeding diets respectively. These results confirm Kp observations in cows, where rate of

passage increases as DMI increases (Colucci et al., 1982; Shaver et al., 1986), and

suggests that precision feeding diets results in a lower rate of passage than ad-libitum

feeding diets. This is also in agreement with a previous study with heifers where higher

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DMI increased Kp (Lascano and Heinrichs, 2009). A lower Kp could increase feedstuff

digestibility. That was not the case in this trial, but it is necessary to complete more

studies comparing ad-libitum vs. precision feeding diets in dairy heifers to further explore

the impacts of Kp on digestibility. If lower Kp increases feedstuff digestibility, it would

be another benefit to precision feeding. Dry matter Kp had a tendency to be greater for

HFQ diets (P = 0.10). Turnover rate of NDF was increased in diets with HNDF levels (P

= 0.04). Turnover rate of iNDF was greater for diets containing HFQ and HNDF (P ≤

0.01 for both). The rate of digestion (Kd) of NDF showed a tendency to be decreased in

precision-fed diets (P = 0.08). Furthermore, diets containing HFQ had lower NDF Kd (P

≤ 0.01).

The turnover times justified our results for turnover rates. Dry matter, NDF, and

iNDF turnover time was much quicker for ad-libitum diets (P ≤ 0.01 for all of them).

These results are in agreement with results presented previously and reinforce that ad-

libitum diets result in lower retention times for the nutrients than that of precision feeding

diets. This is likely due to increased DMI for ad-libitum diets. Diets containing LFQ

showed a tendency to increase DM turnover time (P = 0.09). Turnover time for NDF was

greater for LNDF diets (P = 0.05), and iNDF turnover time was longer for LFQ and

LNDF (P ≤ 0.01 and P = 0.01 respectively).

Fluid rate of passage is presented in Table 8. Fluid volume was not affected by

treatment but showed a tendency to be increased for precision-fed diets and HNDF (P =

0.10 and 0.09 respectively). Increased water consumption for precision-fed heifers,

shown by increased urine production, justifies the tendency for increased rumen fluid

90

volume. Also, an interaction between forage quality and NDF level tended to affect

rumen volume where HFQ-HNDF diets presented higher volumes, but LFQ-HNDF diets

did not show the same results. The dilution rate or fluid rate of passage (Kl), was affected

by the type of diet and forage quality. Ad-libitum diets presented a higher Kl than the

precision feeding diets (10.4 vs. 8.6 %/h; P = 0.04). Also, LFQ presented a higher Kl than

HFQ diets (P ≤ 0.01). Fluid flow rate only showed a tendency to be higher in LFQ than

HFQ diets (P = 0.06). Thus, with the volumes and Kl presented, turnover time was

affected by the type of diet and forage quality. Precision-fed diets showed higher

retention time for fluids in the rumen than ad-libitum diets (11.5 vs. 9.8 h; P = 0.02).

Also, HFQ diets retained fluids longer than LFQ (P ≤ 0.01). This could be explained by

the physical structure of corn silage that allows water to be retained between the fibers of

the stalks. Also, the type of diet and forage quality tended to interact (P = 0.08).

Precision-fed heifers typically had higher turnover times, but an ad-libitum-HFQ diet had

a greater turnover time than both precision-LFQ diets.

Overall, results of rumen volume, Kl, outflow, and turnover time in the rumen are

very similar to results presented in a previous heifer study (Clark and Petersen, 1988) as

well as previous studies with steers (Malcolm and Kiesling, 1990; Okine et al., 1989) and

dairy cows fed corn silage diets (Beauchemin and Yang, 2005). However, our results

were higher than (Bartocci et al., 1997) who evaluated fluid rate of passage in different

species. In this study, type of diet and forage quality were primarily responsible for

differences in rumen fluid and digesta passage.

91

Conclusions

In this study we showed that the reduction in DMI for precision feeding diets

improved feed efficiency in comparison with ad-libitum diets for dairy heifers. We also

found that HFQ diets increased DMI and in an opposite way HNDF diets reduced DMI,

resulting in modified feed efficiency due to changes in intake based on fiber intake

regulation. Thus, data presented in this study indicate that greatest feed efficiency was

obtained by heifers precision fed HFQ-LNDF diets.

Precision-fed diets had a lower minimum pH than ad-libitum diets, but the amount of

time spent at the minimum pH was not enough to modify fiber digestion or rumen

fermentation. Ad-libitum diets had lower mean pH than precision-fed diets, but pH was

more homogeneous throughout the day than in precision feeding, where rapid

fermentation produced by consumption of the whole meal in a couple of hours greatly

reduced pH between 3 and 8 h after feeding. This effect was clearer when corn silage was

the forage source. Also, observations that HNDF diets presented higher minimum pH

suggests that the presence of fiber stimulates rumination and produces the same effect as

a buffer in the rumen. Overall, VFA proportions were not affected by the type of diet but

were clearly modified by forage quality, where grass hay diets presented higher

proportions of acetate and corn silage diets presented higher proportions of propionate.

Heifers consuming precision-fed diets excreted more urine and as a result had greater

total manure excretion. These results lead us to interpret that precision-fed heifers

consumed more water, in part due to confinement during the study as well as lower DMI,

as is described in other studies. Overall, apparent total tract digestibility was not affected

92

by the type of diet. However, DM digestibility increased with HFQ and decreased with

HNDF level. In situ digestibility was affected by forage quality and NDF level, where

grass hay diets presented a higher 48 h digestibility than corn silage. Also, LNDF diets

presented better digestion performance at 30 and 48 h after feeding.

Rate of passage was not affected by type of diet 22 h after feeding, but it was highly

affected with the rumen at maximum capacity, 3 to 4 h after feeding. In this study, ad-

libitum diets had a higher Kp than precision diets for the nutrients analyzed, which could

lead to decreased nutrient digestion. However, that was not the case in this study. More

research is required to evaluate the impacts of Kp on nutrient digestion. Rate of digestion

was affected by forage quality in the post-feeding evaluation, indicating that corn silage

diets presented better Kd than grass hay diets. This suggests that higher amounts of iNDF

reduced the digestion capacity of the rumen. With the results obtained in this study we

can state that the retention time for precision-fed diets was higher than ad-libitum diets

and could lead a better rumen digestion of nutrients. Also, grass hay diets had a higher

retention time than that of corn silage diets. This effect was more significant in the

precision-fed heifers. In addition, Kl or fluid dilution rate was higher for the ad-libitum

diets. Also, the grass hay diets presented a higher Kl than corn silage based diets.

In summary, the 3 factors analyzed in this study affect in greater or lesser extent

ruminal fermentation, rumen pH, nutrient digestion, and rate of passage, but the most

important result was the difference in feed efficiency in the precision feeding diets that

could lead to a reduction in the cost of raising dairy heifers.

Table 5-1. Ingredients and chemical composition of diets with high forage quality (HFQ)

or low forage quality (LFQ) and high NDF (HNDF) or low NDF (LNDF)

93

1 Slow-release urea (Alltech, Nicholasville, KY, USA) contains 96.8% DM and 269.9% CP. 2 Mineral mix, (US Feeds Inc., Eldora, IA) contains 94.28% DM, 11.65% CP, 1.7% soluble CP, 5.46% RUP, 8.29%

ADF, 19.2% NDF, 5.5% fat, 12.4% Ca, 0.36% P, 2.63% Mg, 0.44% K, 0.39% S, 1,628.87 mg/kg Mn, 542.71 mg/kg

Cu, 1,639 mg/kg Zn, 232.94 mg/kg Fe, 9.90 mg/kg Se, 9.2 mg/kg Co, 22.2 mg/kg I, 70.748 IU/g vitamin A, 17.637

IU/g vitamin D and 1.230 IU/g vitamin E. 3 Hemicellulose = NDF – ADF. 4 ME: calculated as TDN × 0.04409 × 0.82. 5ME: Mcal/kg metabolic body weight. 6 Measured with Penn State Particle Separator.

Item

Ad-libitum diets Precision-fed diets

LFQ-HNDF

LFQ-LNDF

HFQ-HNDF

HFQ-LNDF

LFQ-

HNDF LFQ-LNDF

HFQ-HNDF

HFQ-LNDF

Ingredients, % DM

Grass hay 60.00 60.00 -- -- 60.00 60.00 -- --

Corn silage -- -- 60.00 60.00 -- -- 60.00 60.00

Wheat middling 4.00 -- 2.00 18.30 1.05 -- 1.00 14.05

Ground corn 25.40 36.90 -- 5.20 26.50 36.90 -- 7.10

Cotton hulls 7.55 -- 33.00 12.45 9.00 -- 32.25 14.60

Canola meal -- -- -- -- 0.25 -- 2.00 --

Optigen1 0.35 0.40 2.30 1.35 0.50 0.40 2.05 1.55

Sodium chloride 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00

Mineral mix2 1.70 1.70 1.70 1.70 1.70 1.70 1.70 1.70

Chemical composition

DM, % 71.09 71.22 48.28 47.41 70.08 69.35 48.12 48.82

CP, % DM 12.71 12.76 12.80 12.74 12.80 12.76 12.75 12.98

RDP, % CP 21.22 18.21 17.64 22.48 18.43 18.21 17.07 21.22

NDF, % DM 47.68 40.54 48.43 38.99 47.88 40.54 48.10 39.00

ADF, % DM 29.45 23.67 32.51 23.27 29.97 23.67 31.83 23.70

Hemicellulose, % DM3 18.23 16.87 15.92 15.72 17.91 16.87 16.27 15.3

Starch, % DM 18.32 25.30 22.00 29.42 18.36 25.30 21.86 29.60

peNDF, % DM 37.45 29.55 40.39 27.09 34.41 27.68 39.79 26.35

Ash, % DM 6.42 6.66 5.01 5.46 6.81 6.52 5.40 5.33

ME, Mcal/kg DM4 2.26 2.46 2.03 2.42 2.24 2.46 2.03 2.39

ME5, Mcal/kg EBW0.75 0.23 0.24 0.23 0.28 0.21 0.21 0.21 0.21

Ca, % DM 0.53 0.52 0.39 0.37 0.54 0.50 0.41 0.38

P, % DM 0.26 0.25 0.21 0.38 0.24 0.24 0.22 0.35

Na, % DM 0.53 0.52 0.49 0.48 0.52 0.50 0.47 0.49

K, % DM 1.44 1.43 0.95 0.94 1.41 1.41 0.94 0.91

S, % DM 0.19 0.21 0.12 0.14 0.18 0.20 0.13 0.14

Particle size6

>19.0 mm 61.75 55.95 1.62 2.21 52.72 46.51 1.38 1.40

19.0-8.0 mm 6.67 10.36 63.98 50.69 9.79 11.77 62.05 47.21

8.0-4.0 mm 10.12 6.58 17.79 16.58 9.61 10.00 19.03 18.96

< 4.0 mm 21.46 27.10 16.60 30.51 27.87 21.74 17.27 32.43

94

Table 5-2. Body weight, intakes, and feed efficiency in ad-libitum (A-L) vs. precision-

fed (P-F) heifer diets with high forage quality (HFQ) or low forage quality (LFQ) and

high NDF (HNDF) or low NDF (LNDF)

Item

P-Value5

Diet LFQ-

HNDF

LFQ-

LNDF

HFQ-

HNDF

HFQ-

LNDF SE Diet

Forage

quality

NDF

level D × F

D ×

NDF

F ×

NDF

BW, kg1

A-L 512.9 504.4 511.0 510.2 12.70 0.30 0.85 0.15 0.59 0.83 0.27

P-F 493.3 487.6 490.1 488.9

DMI, kg/d A-L 11.0 10.3 12.6 12.4 0.49 ≤0.01 ≤0.01 ≤0.01 0.07 0.21 0.85

P-F 9.7 8.7 10.6 9.3

DMI, % BW A-L 2.14 2.04 2.46 2.44 0.08 ≤0.01 ≤0.01 0.02 0.07 0.17 0.96

P-F 1.96 1.78 2.16 1.89

ADG, kg/d A-L 1.00 1.12 1.19 1.11 0.07 0.15 0.08 0.23 0.33 0.73 0.30

P-F 1.04 1.13 1.12 1.18

Feed efficiency2 A-L 10.98 9.13 10.56 11.14 0.62 ≤0.01 0.24 ≤0.01 0.03 0.91 0.25

P-F 9.33 7.77 9.46 7.81

NDF, kg/d A-L 5.22 4.17 6.08 4.82 0.24 0.01 ≤0.01 ≤0.01 0.08 0.54 0.24

P-F 4.63 3.54 5.10 3.54

iNDF, kg/d A-L 1.50 1.14 1.82 1.21 0.08 ≤0.01 0.04 ≤0.01 0.27 0.77 0.04

P-F 1.38 0.98 1.57 0.91

PdNDF, kg/d A-L 3.72 3.03 4.25 3.61 0.21 0.02 ≤0.01 ≤0.01 0.12 0.56 0.73

P-F 3.26 2.56 3.54 2.63

ADF, kg/d A-L 3.23 2.43 4.27 2.86 0.17 0.02 ≤0.01 ≤0.01 0.12 0.93 0.01

P-F 2.90 2.06 3.58 2.19

Hemicellulose,3 kg/d A-L 1.99 1.74 1.81 1.96 0.09 0.02 0.18 0.02 0.10 0.15 0.03

P-F 1.73 1.47 1.52 1.36

Starch, kg/d A-L 2.00 2.60 2.76 2.76 0.13 0.02 ≤0.01 ≤0.01 0.02 0.03 0.31

P-F 1.78 2.21 2.34 2.76

CP, kg/d A-L 1.53 1.31 1.40 1.27 0.07 0.43 0.03 0.29 0.57 0.02 0.50

P-F 1.37 1.42 1.21 1.30

ME, Mcal/d4 A-L 24.58 24.00 23.66 22.95 1.10 0.48 0.01 0.26 0.14 0.75 0.81

P-F 26.84 25.27 23.02 22.32

1 Average BW for the experiment. 2 Kg of DMI /kg of ADG. 3Hemicellulose = NDF – ADF. 4ME: calculated as TDN × 0.04409 × 0.82. 5D = Diet; F = Forage quality; NDF = NDF level.

95

Table 5-3. Rumen pH, eating time, rate of eating, and VFA, in ad-libitum (A-L) vs.

precision-fed (P-F) heifer diets with high forage quality (HFQ) or low forage quality

(LFQ) and high NDF (HNDF) or low NDF (LNDF)

Item

P-Value1

Diet LFQ-

HNDF

LFQ-

LNDF

HFQ-

HNDF

HFQ-

LNDF SE Diet

Forage

quality

NDF

level D × F

D ×

NDF

F ×

NDF

Daily pH

Mean A-L 6.41 6.33 6.41 6.33 0.06 ≤0.01 0.60 0.04 0.58 0.95 0.76

P-F 6.65 6.59 6.63 6.52

Max A-L 6.98 6.9 6.83 6.85 0.09 ≤0.01 0.84 0.69 0.16 0.99 0.68

P-F 7.22 7.3 7.4 7.23

Min A-L 5.97 5.68 5.83 5.58 0.12 0.05 ≤0.01 0.06 ≤0.01 0.19 0.77

P-F 5.98 5.9 5.25 5.23

Time with feeds2, h/d A-L 24.0 24.0 24.0 24.0 0.66 ≤0.01 0.17 0.93 0.17 0.93 0.90

P-F 3.7 3.5 4.9 4.9

Rate of eating, kg/h A-L 0.46 0.43 0.52 0.52 0.43 ≤0.01 0.43 0.62 0.29 0.58 0.84

P-F 2.83 3.03 2.19 2.59

Total VFA (mM) A-L 90.88 117.98 111.03 110.07 7.02 0.05 0.96 0.28 0.21 0.13 0.32

P-F 101.48 95.21 90.76 92.77

Individual VFA (% of mM)

Acetate A-L 65.90 64.38 62.99 60.36 0.60 0.17 ≤0.01 ≤0.01 ≤0.01 0.89 0.10

P-F 66.13 64.73 60.88 58.31

Propionate A-L 16.79 17.47 20.43 21.89 0.68 0.52 ≤0.01 0.60 0.17 ≤0.01 0.07

P-F 17.87 18.42 22.73 19.06

Butyrate A-L 12.46 12.95 12.25 12.30 0.49 0.99 0.02 ≤0.01 ≤0.01 ≤0.01 ≤0.01

P-F 10.92 11.12 11.62 16.28

Isobutyrate A-L 1.44 1.50 1.41 1.45 0.05 0.04 0.52 0.48 0.60 0.40 0.93

P-F 1.55 1.60 1.66 1.56

Valerate A-L 1.53 1.67 1.94 1.93 0.80 ≤0.01 ≤0.01 0.36 0.69 0.72 0.49

P-F 1.43 1.46 1.80 1.83

Isovalerate A-L 1.86 2.04 1.89 2.07 0.09 0.04 0.44 0.83 0.23 0.01 0.10

P-F 2.13 2.20 2.23 1.85

Acetate:propionate A-L 3.95 3.72 3.19 2.83 0.12 0.29 ≤0.01 0.21 0.21 0.02 0.24

P-F 3.69 3.54 2.81 3.17

1 D = Diet; F = Forage quality; NDF = NDF level. 2 Time with feed in the feed bunk

96

Table 5-4. Excretion parameters in ad-libitum (A-L) vs. precision-fed (P-F) heifer diets

with high forage quality (HFQ) or low forage quality (LFQ) and high NDF (HNDF) or

low NDF (LNDF)

Item

P-Value2

Diet LFQ-

HNDF

LFQ-

LNDF

HFQ-

HNDF

HFQ-

LNDF SE Diet

Forage

quality

NDF

level D × F

D ×

NDF

F ×

NDF

Wet feces, kg/d A-L 17.2 15.6 16.9 16.7 0.98 0.89 0.20 0.23 0.52 0.91 0.44

P-F 16.6 15.6 17.6 17.1

Dry feces, kg/d A-L 4.3 3.8 4.4 3.6 0.33 0.10 0.62 0.02 0.82 0.82 0.33

P-F 3.9 3.3 4.0 2.9

Urine, kg/d A-L 9.3 8.8 9.5 8.2 2.23 0.02 0.20 0.18 0.14 0.66 0.92

P-F 16.8 14.4 19.1 17.9

Total manure1, kg/d A-L 26.5 24.4 26.4 24.9 2.59 0.03 0.11 0.11 0.15 0.78 0.65

P-F 33.4 30.0 36.6 35.0

1 Feces included on as-is (wet) basis. 2 D = Diet; F = Forage quality; NDF = NDF level.

97

Table 5-5. Apparent total tract nutrient digestibility and in situ digestibility in ad-libitum

(A-L) vs. precision-fed (P-F) heifer diets with high forage quality (HFQ) or low forage

quality (LFQ) and high NDF (HNDF) or low NDF (LNDF)

Item

P-Value2

Diet LFQ-

HNDF

LFQ-

LNDF

HFQ-

HNDF

HFQ-

LNDF SE Diet

Forage

quality

NDF

level D × F

D ×

NDF

F ×

NDF

Digestibility, % DM

DM A-L 60.6 63.5 65.7 71.0 2.72 0.41 ≤0.01 0.02 0.48 0.91 0.28

P-F 59.7 61.9 61.4 68.2

Starch A-L 98.9 98.8 99.3 99.5 0.14 0.01 ≤0.01 0.26 0.71 0.79 0.44

P-F 98.4 98.7 99.3 99.1

NDF A-L 47.4 45.8 46.8 49.8 4.57 0.35 0.85 0.81 0.41 0.99 0.32

P-F 45.4 43.0 41.5 43.3

ADF A-L 36.4 32.8 42.5 41.2 4.03 0.29 0.03 0.35 0.69 0.97 0.38

P-F 34.8 28.6 36.4 37.2

Hemicellulose A-L 65.3 63.7 57.3 62.6 6.90 0.43 0.05 0.59 0.32 0.90 0.46

P-F 63.3 63.3 47.3 53.0

24-h DMD1 A-L 61.5 60.9 52.7 64.7 1.72 0.93 0.01 ≤0.01 0.42 0.55 ≤0.01

P-F 60.8 63.8 52.1 63.5

30-h DMD A-L 62.5 66.6 57.2 67.9 1.25 0.68 ≤0.01 ≤0.01 0.03 0.21 0.03

P-F 61.9 70.7 54.7 65.3

48-h DMD A-L 73.8 78.8 65.0 74.1 1.43 0.81 ≤0.01 ≤0.01 0.64 0.95 0.32

P-F 72.5 79.7 66.6 73.8

1 Dry matter digestibility calculated in base of in situ bags digestion. 2 D = Diet; F = Forage quality; NDF = NDF level.

98

Table 5-6. Pre-feeding rumen digestion kinetics in ad-libitum (A-L) vs. precision-fed (P-

F) heifer diets with high forage quality (HFQ) or low forage quality (LFQ) and high NDF

(HNDF) or low NDF (LNDF)

Item

P-Value1

Diet LFQ-

HNDF

LFQ-

LNDF

HFQ-

HNDF

HFQ-

LNDF SE Diet

Forage

quality

NDF

level D × F

D ×

NDF

F ×

NDF

Rumen volume, L A-L 73.04 71.11 80.12 72.40 4.99 0.47 0.29 ≤0.01 ≤0.01 0.81 0.08

P-F 74.33 72.07 69.82 60.81

Rumen mass, kg A-L 53.51 56.19 62.73 57.35 4.55 0.96 0.93 0.05 ≤0.01 0.24 0.05

P-F 61.15 58.64 58.45 50.40

Rumen density, kg/L A-L 0.73 0.79 0.78 0.80 0.02 0.04 0.10 0.24 0.53 0.08 0.32

P-F 0.82 0.82 0.84 0.83

Pool DM, % A-L 13.92 13.22 16.25 14.46 0.66 0.08 ≤0.01 ≤0.01 0.39 0.26 0.02

P-F 12.37 11.75 16.59 12.66

Pool mass, kg of DM A-L 7.43 7.42 10.29 8.33 0.83 0.49 ≤0.01 ≤0.01 0.15 0.19 ≤0.01

P-F 7.53 6.91 9.69 6.39

NDF pool, % A-L 69.12 69.15 77.77 71.60 1.37 0.98 ≤0.01 0.01 0.56 0.75 0.04

P-F 69.30 67.90 77.01 73.52

NDF pool, kg of DM A-L 5.14 5.14 8.06 5.94 0.66 0.50 ≤0.01 ≤0.01 0.23 0.35 ≤0.01 P-F 5.22 4.70 7.45 4.70

iNDF pool, % A-L 27.59 27.82 26.61 23.92 1.48 0.47 ≤0.01 0.06 0.57 0.36 0.60

P-F 30.69 27.46 26.70 23.86

iNDF pool, kg of DM A-L 3.84 3.66 4.28 3.47 0.30 0.50 0.41 ≤0.01 0.83 0.23 0.08

P-F 3.81 3.25 4.43 3.03

pdNDF pool, kg of DM A-L 1.79 1.48 3.78 2.47 0.51 0.40 ≤0.01 0.02 0.33 0.79 0.06

P-F 1.41 1.46 3.02 1.67

Digestible fraction % A-L 71.17 72.81 69.92 74.96 1.62 0.63 0.79 ≤0.01 0.90 0.93 0.19

P-F 70.51 72.63 69.25 74.22

NDF rumen digestion % A-L 58.06 61.84 44.12 57.05 5.02 0.81 0.03 0.09 0.81 0.62 0.25

P-F 59.64 60.27 47.93 56.75

Turnover, %/h2

DM A-L 9.4 8.53 6.8 9.0 0.07 0.26 0.05 0.03 0.66 0.50 ≤0.01

P-F 7.7 7.9 5.9 8.3

NDF A-L 4.5 3.5 3.3 3.5 0.03 0.24 0.02 0.18 0.65 0.42 0.01

P-F 3.7 3.2 2.9 3.2

pdNDF A-L 8.4 7.6 5.5 7.0 0.21 0.53 0.23 0.43 0.83 0.57 0.06

P-F 9.8 7.9 5.1 10.2

iNDF A-L 1.6 1.3 1.8 1.5 0.01 0.08 0.28 0.01 0.58 0.54 0.85

P-F 1.5 1.2 1.5 1.3

Kd-1, NDF3 A-L 6.8 6.3 3.8 5.5 0.19 0.49 0.23 0.33 0.81 0.64 0.08

P-F 8.3 6.7 3.6 8.9

Turnover time, h

DM A-L 11.16 11.94 15.21 11.47 1.14 0.30 0.01 ≤0.01 0.95 0.50 ≤0.01

P-F 12.95 12.99 17.04 12.32

NDF A-L 23.44 29.60 31.39 29.91 2.58 0.29 ≤0.01 0.27 0.87 0.67 0.01

P-F 27.03 31.98 35.45 32.48

iNDF A-L 61.58 80.78 56.28 68.84 7.22 0.12 0.43 0.01 0.39 0.76 0.65

P-F 67.82 81.91 69.59 80.87

99

1 D = Diet; F = Forage quality; NDF = NDF level. 2 Fractional rate of passage. 3 Fractional rate of digestion, %/h.

100

Table 5-7. Post-feeding rumen digestion kinetics in ad-libitum (A-F) vs. precision-fed

(P-F) heifer diets with high forage quality (HFQ) or low forage quality (LFQ) and high

NDF (HNDF) or low NDF (LNDF)

Item

P-Value1

Diet LFQ-

HNDF

LFQ-

LNDF

HFQ-

HNDF

HFQ-

LNDF SE Diet

Forage

quality

NDF

level D × F

D ×

NDF

F ×

NDF

Rumen volume, L A-L 83.34 83.66 80.44 77.22 5.76 0.05 0.01 0.09 0.14 0.20 0.98

P-F 109.08 97.82 93.31 85.91

Rumen mass, kg A-L 66.99 65.73 64.88 62.22 6.32 0.04 0.01 0.73 0.04 0.86 0.28

P-F 88.19 95.06 76.67 68.53

Rumen density, kg/L A-L 0.81 0.79 0.81 0.81 0.05 0.19 0.34 0.36 0.18 0.27 0.25

P-F 0.81 0.97 0.82 0.80

Pool DM, % A-L 16.80 16.91 17.14 17.87 0.83 0.71 0.28 0.59 0.98 0.22 0.03

P-F 18.34 14.90 16.63 17.95

Pool mass, kg of DM A-L 11.19 11.17 11.11 11.17 1.20 0.04 0.11 0.42 0.12 0.41 0.63

P-F 16.13 14.13 12.87 12.29

NDF pool, % A-L 60.49 55.73 70.05 61.89 1.48 0.16 ≤0.01 ≤0.01 0.09 0.26 0.38

P-F 60.36 56.50 64.73 60.47

NDF pool, kg of DM A-L 6.78 6.23 7.78 6.91 0.74 0.08 0.81 0.04 0.05 0.56 0.80 P-F 9.73 8.08 8.26 7.42

iNDF pool, % A-L 21.97 24.63 22.73 19.86 0.90 0.30 0.01 ≤0.01 0.69 0.48 0.73

P-F 25.00 22.59 22.96 21.64

iNDF pool, kg of DM A-L 2.78 2.44 2.53 2.22 0.34 0.04 0.03 0.08 0.19 0.66 0.62

P-F 4.03 3.30 2.98 2.67

pdNDF pool, kg of DM A-L 4.01 3.79 5.27 4.69 0.43 0.17 0.10 0.04 0.02 0.51 0.98

P-F 5.70 4.78 5.28 4.75

Digestible fraction, % A-L 71.17 72.81 69.92 75.96 1.63 0.63 0.79 0.01 0.13 0.93 0.19

P-F 70.51 72.63 69.25 74.22

NDF rumen digestion, % A-L 29.27 31.11 11.05 21.62 3.88 0.27 ≤0.01 0.05 0.23 0.97 0.26

P-F 28.07 31.75 19.04 27.29

Turnover, %/h2

DM A-L 6.8 7.0 6.8 7.8 0.005 ≤0.01 0.10 0.26 0.50 0.59 0.98

P-F 4.2 4.8 5.5 5.3

NDF A-L 3.2 2.8 3.3 3.0 0.002 ≤0.01 0.12 0.04 0.43 0.96 0.51

P-F 2.0 1.9 2.6 2.0

pdNDF A-L 3.9 3.4 3.4 3.4 0.002 ≤0.01 0.86 0.15 0.21 0.96 0.95

P-F 2.4 2.3 2.8 2.3

iNDF A-L 2.3 1.9 3.1 2.4 0.002 ≤0.01 ≤0.01 ≤0.01 0.86 0.97 0.07

P-F 1.5 1.3 2.4 1.5

Kd-1, NDF3 A-L 1.6 1.5 0.5 1.0 0.002 0.08 ≤0.01 0.33 0.08 0.88 0.26

P-F 1.0 1.0 0.7 0.9

Turnover time, h

DM A-L 14.88 14.48 14.88 13.38 1.76 ≤0.01 0.09 0.54 0.19 0.88 0.73

P-F 24.14 22.16 18.87 19.70

NDF A-L 31.14 35.60 30.69 34.58 4.24 ≤0.01 0.21 0.05 0.30 0.52 0.48

P-F 50.70 54.15 39.18 51.69

iNDF A-L 44.27 52.19 33.24 44.07 6.33 ≤0.01 ≤0.01 0.01 0.43 0.52 0.20

P-F 73.66 78.46 46.55 72.11

101

1 D = Diet; F = Forage quality; NDF = NDF level. 2 Fractional rate of passage. 3 Fractional rate of digestion %/h.

102

Table 5-8. Fluid passage rate in ad-libitum (A-F) vs. precision-fed (P-F) heifer diets with

high forage quality (HFQ) or low forage quality (LFQ) and high NDF (HNDF) or low

NDF (LNDF)

Item

P-Value1

Diet LFQ-

HNDF

LFQ-

LNDF

HFQ-

HNDF

HFQ-

LNDF SE Diet

Forage

quality

NDF

level D × F

D ×

NDF

F ×

NDF

Fluid volume, L A-L 57.0 60.0 68.3 54.4 4.55 0.10 0.21 0.09 0.93 0.63 0.09

P-F 68.8 65.8 72.2 68.9

Fluid dilution rate, %/h2 A-L 11.1 10.7 9.4 10.5 0.006 0.04 ≤0.01 0.75 0.66 0.22 0.33

P-F 9.5 9.1 8.4 7.7

Fluid flow rate, L/h A-L 6.2 6.3 6.4 5.7 0.33 0.65 0.06 0.13 0.30 0.73 0.17

P-F 6.5 6.3 6.0 5.3

Fluid turnover time, h A-L 9.2 9.5 10.2 9.7 0.64 0.02 ≤0.01 0.98 0.08 0.27 0.92

P-F 10.6 10.3 12.7 13.0

1 D = Diet; F = Forage quality; NDF = NDF level. 2 Fractional rate of passage of fluid.

103

♦ with solid line indicates HFQ-HNDF; ■ with dotted line indicates HFQ-LNDF; ▲

with dashed line indicates LFQ-HNDF; ● with dash-dot line indicates LFQ-LNDF.

Figure 5-1. Rumen pH and total VFA production over 24 h in ad-libitum (left

column) vs. precision-fed (right column) heifer diets with high forage quality (HFQ)

or low forage quality (LFQ) and high NDF (HNDF) or low NDF (LNDF).

104

♦ with solid line indicates HFQ-HNDF; ■ with dotted line indicates HFQ-LNDF; ▲ with dashed

line indicates LFQ-HNDF; ● with dash-dot line indicates LFQ-LNDF.

Figure 5-2. Fermentation end products over 24 h in ad-libitum (left column) vs.

precision-fed (right column) heifer diets with high forage quality (HFQ) or low

forage quality (LFQ) and high NDF (HNDF) or low NDF (LNDF).

106

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Chapter 6

Summary and conclusions

Based on the literature reviewed and the studies done in this thesis, precision-

feeding systems for dairy heifers are nutrient efficient, reduce costs, and improve feed

efficiency without affecting health or any other known parameter of the animals. Rumen

fermentation and pH changes are important aspects that we monitor in dairy heifer

nutrition studies. Based on our experiments, there is no risk of rumen acidosis due to the

large proportion of dietary concentrates. As the total amount of rapidly fermented

carbohydrates is reduced in a precision- or limit-fed heifer diet (due to limited intake),

fermentation is faster than in an all forage diet. However, the decline in rumen pH is not

large enough to produce rumen acidosis in these limit-fed heifers. It also appears that this

sudden decline in rumen pH from our higher concentrate diets is not large enough to be

detrimental for the continued growth of fibrolytic bacteria. In the 3 experiments presented

in this thesis, none showed depressed fiber digestibility when concentrate or starch level

increased in the diet. Also when a depression in fiber digestion occurs, acetate production

also decreases at the expense of propionate and butyrate. Rumen acetate production is not

extremely important to the heifer, as it is in the lactating dairy cow that uses acetic acid

for milk fat production, and energetic metabolism can be sustained in the heifer by

propionate and butyrate. In fact, energy production is slightly more efficient in this

situation because of the way propionate and butyrate are metabolized in the liver.

Concerning the effect of TM on rumen function, more research is needed to

further understand their impact in the dairy heifer. Even though they are not very

120

abundant in the rumen, they could have a large impact on rumen bacteria populations.

The better palatability of OTM, showed in the time spent in consuming the whole ration,

and the better bioavailability of OTM in the rumen could impact rumen fermentation.

Thus, OTM showed faster rumen fermentation, changing rumen pH, modifying the VFA

proportions, and significantly increasing total VFA and butyrate production when heifers

received OTM diets. More research is necessary to evaluate and understand how

microbes use some of these TM. Another important finding is that a large period of

adaptation to TM feeding is necessary to observe changes. The first study in this thesis

showed that enzymatic activity of superoxide dismutase, ceruloplasmin, and glutathione

peroxidase were not different between inorganic and organic sources in a Latin square

design; however, we analyzed the variation over time and observed an increase in

enzymatic activity of glutathione peroxidase and superoxide dismutase that was greater

for the organic than inorganic source of TM at the end of the trial (data not presented in

the paper). Thus, we showed that TM need time to produce changes, especially changes

in gene expression of enzymes. Also, mineral intake was lower in OTM, but plasma

concentrations did not show differences from heifers fed ITM, which suggests that OTM

presented better absorption than ITM.

A point that was observed in the 3 trials was the lack of heifer vocalizations in

precision-fed diets. In our studies, all the precision-fed diets were consumed within 4 h

after feeding, but heifers never presented alterations in behavior or vocalizations.

Diets used in these studies with higher proportions of concentrates or rapidly

fermentable carbohydrates resulted in higher feed efficiency. Thus precision feeding

systems improve the efficiency of heifers, which reduces the amount of feed fed and

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increases digestibility, reducing both nutrient losses and manure output. These changes

improve economic outcomes for farmers, making this system of feeding heifers more

profitable than traditional feeding systems.

We used new varieties of sorghum in one study and this was compared with corn

silage for feeding heifers. We observed that corn silage is a slightly better crop source to

feed dairy heifers than sorghum. However, sorghum is an adequate alternative to feed

dairy heifers in a precision feeding system. The study showed good apparent total tract

digestibilities (comparable with corn silage when fed at similar F:C), enough digestible

fiber that can keep rumen pH stable in diets with 45% concentrates (min. pH 5.7), with

good palatability showed by the eating time of the whole ration. Feed efficiency was

comparable to corn silage with the same F:C. Analysis of in situ sorghum degradation

showed similar degradation rates as corn silage, proving that the fiber digestion of new

varieties with a low proportion of indigestible NDF makes sorghum a good alternative for

precision-fed dairy heifers.

Overall, comparing our studies and previous literature, we can say that the best

overall performance based on digestibility, rumen fermentation (pH and VFA

production), and feed efficiency is in precision-fed heifers with a 60:40 F:C. The last

study proved that feed efficiency of precision-fed heifers is very superior to heifers fed ad

libitum. Also, the best feed efficiency was obtained with diets that contained corn silage

with low NDF, principally because of higher retention time in the rumen, higher rates of

nutrient digestibilities, and changes in rumen fermentation.

In general, these studies conclude that feed efficiency can be improved by

precision feeding diets, leading to a real alternative for farmers to reduce heifer rearing

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costs and increase profitability. Still, more information is required about the optimum ME

intake in the different periods of heifer growth, particularly from weaning to breeding, to

help farmers improve the efficiency of dairy heifers in this period of their life.

VITA

Felipe Pino San Martin

ACADEMIC PROFILE

August 2012- PhD Candidate in Animal Science, The Pennsylvania State University

October 2008 Doctor of Veterinary Medicine (Highest Honors), Faculty of Veterinary Medicine,

University of Chile, Chile.

December 2006 B.S. Veterinary Medicine, Faculty of Veterinary Medicine, University of Chile, Chile. PROFFESIONAL EXPERIENCE

2010 – 2012: Technical Chief at consultancy for small agricultural and livestock farmers in Puerto Octay,

Southern Chile, for a Government Program.

September to December 2010: Consultancy in Animal Nutrition at “Champion”.

September 2010: Consultancy in Beef Cattle Production at a Government Program, Talca.

2009-2010: Independent consultant in Animal Production at chilean farms.

2008-2009: Farm manager and veterinary. SELECTED PUBLICATIONS

Pino, F. H., and A. J. Heinrichs. 2014. Comparison of on-farm forage-dry-matter

methods to forced-air oven for determining forage dry matter. The Professional Animal

Scientist 30(1):33-36.

Pino, F., and A. J. Heinrichs. 2016. Effect of trace minerals and starch on digestibility

and rumen fermentation in diets for dairy heifers. J Dairy Sci 99(4):2797-2810.

Pino, F., and A. J. Heinrichs. 2016. Sorghum forage in precision-fed dairy heifers diets.

In Press. Journal of Dairy Science.

Gelsinger, S. L., F. Pino, C. M. Jones, A. M. Gehman, and A. J. Heinrichs. 2016.

Effects of a dietary organic mineral program including mannan oligosaccharides for

pregnant cattle and their calves on calf health and performance. The Professional Animal

Scientist 32(2):205-213.

K. Kliak, F. Pino amd A.J. Heinrichs. 2016. Effect of forage to concentrate ratio with

sorghum silage as a source of forage on rumen fermentation, N-C balance, and purine

derivative excretion in limit-fed dairy heifers. In Press in Journal of Dairy Science.