ingestive behaviour of beef cattle grazing alfalfa (medicago sativa l.)+

Grass and Forage Science (1988) Volume 43, 121-130

Ingestive behaviour of beef cattle grazing alfalfa(Medicago sativa L.)t

C. T. DOUGHERTY, E. M. SMITH,N. W. BRADLEY, T. D. A. FORBES,P. L. CORNELIUS, L. M. LAURIAULTAND C. D. ARNOLDUniversity of Kentucky, Lexington, Kentucky,USA

Abstract

Development of simulation models of grazingbeef cattle requires measurement of the compo-nents of the ingestive process and the establish-ment of relationships between these componentsand the structure of the sward. The ingestivebehaviour of eight half-sib Angus steers (liveweight (LW), ^ = 270 kg) grazing alfalfa {Medi-cago sativa L.) was studied at three stages ofmaturity (26, 40 and 47 days of regrowth) and atfour allowances of herbage dry matter (DM) (1 -0,1-5, 20 and 2-5 kg per 100 kg LW) at each of twodaily grazing sessions. A tethering system ofgrazing was used in which the experimental unitwas a tethered steer and its plot for one grazingsession. Grazing sessions commenced at 08.00and 14.00 h EDT. Intake (DM) increased linearlyfrom 1 -98 kg per steer session at a DM allowanceof 1 kg (100 kg LW)- ' to 2 89 kg steer session atan allowance of 2-5 kg (100 kg LW)-' as utiliza-tion of herbage declined linearly from 0-69 to0-43. Herbage DM intake per bite increased from1 0 g at 1 kg (100 kg LW)-' allowance to 1-5 g at2-5 kg (100 kg LW)- ' allowance. Rates of bitingwere not affected by herbage allowance andaveraged 21 bites min-'. Dry matter intake

Correspondence: Dr C. T. Dougherty, Department ofAgronomy, University of Kentucky, Lexington, Kentucky,40546-0091 USA.

t The investigation reported in this paper (No. 86-3-203) is inconnection with a project of the Kentucky Agricultural Ex-periment Station and is published with the approval of theDirector.

increased from 1 -77 to 3 41 kg per steer session asthe alfalfa matured and herbage mass changedfrom 1500 to 4656 kg ha- ' . Mean rates of bitingwere 24 bites min-' for steers grazing the youn-gest alfalfa and 16 bites min-' for steers on theoldest forage. Herbage DM intakes per bite were1-1 g and 1-7 g at the same stages. Rates of DMintake approached 2 kg h" ' and maximum dailyDM intake was estimated at 2 75 kg (100 kgLW)- ' . Intake of alfalfa was limited by allowanceand mass of herbage above a canopy horizon of20 cm and, to a lesser extent, by the length of fast.

Introduction

Grazing of alfalfa is a limited, but increasinglyimportant practice in the USA (Allen et al.,1986a; b), but it is common form of managementon many New Zealand sheep farms (Douglas,1986). Productivity of animals grazing alfalfapastures may be quite high and is dependent onrotational grazing at an appropriate stage ofdevelopment and at non-limiting levels of allow-ance and herbage mass (Douglas, 1986). Theoptimum stage of growth for grazing of alfalfa, interms of productivity and stand longevity, hasbeen established to be similar to that recom-mended for harvesting as hay, silage or green-chop (Douglas, 1986).

Understanding complex grazing systems andthe development of models of these systemsrequires mathematical logic of the ingestive beha-viour of the animals (Forbes et al., 1985). Volun-tary intake of grazing animals is the product ofgrazing time, biting rate and herbage intake perbite (Hodgson, 1982a). Herbage intake per bite isdetermined by the dimensions of the mouth, theforage-gathering capacity of the tongue, thephysical properties of the sward (Forbes et al.,1985) and by the hunger-satiety status of the

121

122 C. T. Dougherty et al.

Table 1. Weather variables and characteristics of alfalfa at threestages of regrowth before grazing

Variable

DatesTemperature, highTemperature, lowRelative humidity

highlow

PrecipitationPan evaporationAgeFlowering

Plot areaHerbage mass (DM)Stem heightCanopy heightStem mass (DM)Leaf mass (DM)

Units

June 1985"C°C

%A>

mmmmDays

Flowers(100 stems)"

m^kg h a " '

mmmm

mg stem"'%

I

4-62619

10070II1125-27

' 131 5150039434728645

Stage

II

18-202414

99530

1839-41

5015 0

286057037364137

III

25-273018

9949

02246-48

>9010 9

465667340695640

grazing animal (Baile and McLaughlin, 1987).The animal compensates for small bites by acce-lerating the rate of biting in an apparent, and onlypartially successful, attempt to maximize the rateof herbage intake (Hodgson, 1977; 1982b). Thetime spent grazing is also variable and is used bythe animal to compensate for low rates of intake.Herbage mass within the grazing horizon is aprimary determinant of intake per bite. Herbagemass may decrease markedly within a grazingsession (Chacon and Stobbs, 1976).

In this paper we present data on the ingestiveprocesses of beef cattle grazing alfalfa in a highlycontrolled and simple system using tetheredanimals in which the experimental unit is anindividual steer and its grazing area as delineatedby the radius of its tether.

Proeedures

Forage management

A 3-year-old stand of alfalfa {Medicago sativa L.cv. Classic) growing on Maury silt loam (fine,mixed, jnesic, Typic Paleudalf) of the MainChance farm of the Kentucky AgriculturalExperiment Station was used in this study. Thefirst crop was harvested for hay on 10 May 1985and three consecutive stages of regrowth of thesecond crop were used as stages of crop maturity(Table 1). The experimental field was divided intonine blocks of equal area used sequentially eachday of grazing. Herbage mass (>5 cm) was

determined each day of measurement on a strip(1-5x90 m) parallel to that day's block with aHaldrup forage harvester (Sheldrick et al., 1985).Samples of harvested forage were oven-dried to aconstant weight at 75°C, Herbage dry matter DMmass allowances for each grazing session werebased on 1-0, 1-5, 2-0 and 2-5% of mean LW (270kg) of steers at the beginning of the experiment.Tether lengths were adjusted at each crop stageaccording to herbage mass and assigned herbageallowances. Each day sixteen steel poles wereplaced in that day's block with distances betweenpoles 1 m longer than twice the longest tetherlength. The sixteen plots for the morning andafternoon grazing sessions were randomlyassigned and delineated by mowing a swath (55cm) around their circumferences with a rotarymower tethered to the centre pole. Non-destruc-tive methods were used to measure canopy andextended stem heights at ten sites within eachplot. Before and after grazing, one small quadrat(0-25 m )̂ was hand-harvested to the soil surface ineach plot to determine DM proportions of leafand stem of alfalfa, dead herbage and weedspecies. Residual herbage mass (RDM) (> 5 cm)in each grazed plot was harvested and weighedwith the forage harvester and samples taken todetermine moisture content (75°C), Grazed plotswere not harvested until all post-grazing swardmeasurements were completed and during theinterval the attitude of trampled stems was par-tially restored. The lifting action of the guards ofthe knife and the action of the brushes of theharvester reel ensured that virtually all trampledstems were severed at 5 cm. Inspection of swardsfollowing grazing revealed no evidence that stemswere grazed below 5 cm. Herbage intake per plotwas estimated as the difference in herbage massbefore and after grazing (Meijs et al., 1982).

Animal management

Eight yearling half-sib Angus steers were halter-broken, trained to lead and graze while tetheredbefore the experiment commenced. Steers werepre-conditioned to alfalfa and the grazing sched-ule for 4 days before the measurement phase.When not grazing, steers were confined to a smallcorral with ad libitum access to water and amineral supplement impregnated with poloxa-lene. Animals also had access to shade and shelterand were treated for control of ecto- and endo-parasites. Between measurement phases steers

Ingestive behaviour of grazing eattle 123

Table 2. ANOVA, expected mean squares and appropriate F-tests

Source d.f. Mean square Expected mean squares F-lest

Among A and D*:RA(R)SS*RS*A(R)D(S)Residual (error a)

M within A and D:MM*RM'A(R)M»SM'S*RM*S*A(R)M*D(S)

Residual (error b)

3426g6

42

1342686

42

MlM2M3M4M J

M6M7

MgM9MioMilM12M|3M,4Mis

(M1/M2)

(M4/M5)

(M9/MI0)

(M,i + M|(M,2/M|3)

* Abbreviations: R, allowances; A, animals; S, stages of growth; D, days; M, grazing sessions; q(M), q(S),etc., represent non-negative quadratic functions of M,S, etc.

were permitted to graze the same alfalfa for up to2 h during two sessions each day on the experi-mental schedule except during inclement weatherwhen alfalfa hay made from the first harvest wasfed in the corral. Herbage mass DM allowancesbetween measurement phases were the same forall steers (4 kg (100 kg LW)-').

Steers were led from the corral to grazing plotsand commenced grazing at 08.00 h and 14.00 hEDT. Grazing times were recorded by observerswho also counted the number of bites taken persteer for 1 min every 10 min during grazing(Hodgson, 1982a). When grazing ceased steerswere returned to the corral. The number of bitesper grazing period was calculated as the productof rate of biting and grazing time. Herbage intakeper bite was derived from the estimate of herbagedry matter intake per grazing period divided bythe number of bites taken per grazing session.

Experimental design and analysis

Sources of variation partitioned in the analyses ofvariance (Table 2) were herbage allowance (R),animals within allowance (A in R), stages ofregrowth (S), S*R, S*A in R, days (D) withinstages (D in S), grazing sessions (M) and theinteractions of M with R, A in R, S, S*R, S*(A inR) and D in S. R*(D in S) and (A*R)*(D in S)were pooled as a single A*D error component(error a) and M*R*(D in S) and M*(A in R)*(Din S) were pooled as a residual error component

(error b). Expected mean squares were deter-mined by the method described by Scheffe (1959)with R, S and M considered fixed and A and Dconsidered random. The resulting expected meansquares did not provide appropriate meansquares against which to test S, M and M*S;consequently, linear combinations of meansquares were used as numerators and denomi-nators of approximate F-statistics; for example, SefTects were tested using [MS(S)-l-MS(error a)]/[MS(S*(A in R))-|-MS(D in S)] (Table 1).Degrees of freedom for numerator and denomi-nator, in this and analogous approximate F-statistics, for tests of M and M*S were derived bythe method of Satterthwaite (1946). The ANOVAprocedure of SAS (1985) was used to computeanalysis of variance for measured variables.Where applicable, means were separated byunprotected least significant differences (LSD)and, where appropriate, significance of linear andquadratic terms were determined (Carmer andWalker, 1985).

Results

Variables describing the alfalfa before grazing arepresented for each stage of growth in Table 1.Properties of the sward following grazing at thesestages and averaged across the allowance andsession treatments are shown in Table 3. Vari-ables describing alfalfa before and after grazing


Table 3. Characteristics of alfalfa following grazing at threestages of regrowth

Stage

Variable Units 1 II III LSDoos

Herbage mass (DM) kg haDry matter %Stem height mmCanopy height mmStem population Tillers m"Stem mass (DM)Leaf mass (DM)

mg stem"

906 1288 1391 82160 241 280 07296 415 462 15194 196 183 15463 326 260 34219 343 458 40

29 12 10 2

for allowance levels across the other treatmentsare presented in Table 4. Data for canopies beforeand after grazing for sessions are not presentedbecause differences were non-significant (/• > 0 05).Main effect means of measured variables andrelevant statistics are given for stages of growth(Table 5), herbage allowances (Table 6) andgrazing sessions (Table 7). Data for some of thevariables involved in significant interactionsbetween stage of growth and herbage allowanceare presented in Table 8.

Stage of growth

When the alfalfa was first grazed the crop was25-27 d old and flowering had commenced.Grazing reduced herbage mass (> 5 cm) from1500 to 905 kg ha- ' , extended stem height from39 to 30 cm, stem DW from 286 to 219 mg stem"'and the leaf DW fraction from 0 45 to 0 29(Tables 2 and 3). Grazing of alfalfa canopies at46-48 d of age and close to full flower reducedherbage mass (> 5 cm) from 4656 to 1391 kg ha"' ,extended stems from 67 to 46 cm, stem DW from956 to 462 mg and leaf fraction from 0 4 to 01(Tables 2 and 3). Alfalfa at 39-41 d of regrowthwas intermediate in canopy characteristics(Tables 2 and 3). Canopy surface heights werereduced to slightly below 20 cm by grazing at allstages (Table 3). Differences between extendedstem height and canopy surface height aftergrazing reflect the combined effects of grazing,lodging and trampling.

Main effect means (Table 5) indicate thatutilization of herbage mass (> 5 cm) and RDM( > 5 cm) increased linearly ( / '<0 01) withadvancing stages. Dry matter intake per steer persession averaged 1-2 kg when they grazed theyoungest alfalfa and 3 4 kg when they grazed theoldest sward. Much of this difference was attribu-

table to grazing time, which was extended from 87min at the 25-27-d stage to 130 min when theywere grazing alfalfa at the two older stages. WhenDM intakes were expressed on an hourly basisdifferences were less,, with rates of 1 38 kg h" 'recorded for steers grazing the youngest materialand 1-64 kg h" ' when they grazed the oldestmaterial. With the development of the alfalfacanopy herbage DM intake per bite increasedlinearly from 11 g to 17 g per bite, whileconcurrently, rates of biting declined from 24 to16 bites

Herbage allowance

Herbage allowance modified ingestive behaviourof steers and, consequently, properties of alfalfaswards after grazing (Table 6). Measures ofherbage intake were involved in significant inter-actions between stage of growth and herbageallowance and will be discussed in a later section.Herbage DM intake per steer session increasedlinearly from 1 98 to 2 89 kg as DM allowances( > 5 cm) increased from 10 to 2 5 kg (100 kgLW)-'. Since grazing time was not greatlyaffected by herbage allowance, rates of DMintake followed similar trends and increased from1-1 to 18 kg h"'. Concurrently, herbage DMutilization declined from 0 69 to 0 44 and RDMincreased from 732 to 1550 kg ha"' . Intake ofstem tissue DM across allowances was constantat just over 1 kg per steer session. The effect ofallowance was mainly expressed in consumptionof leaf tissue. Grazing time was 124 min persession for steers assigned to the 1-5 kg (100 kgLW)- ' allowance and declined with increasingallowance to 106 min for steers on the largestherbage allowance. Steers assigned to 10 kg(100kg LW)- ' allowance grazed for 113 min.Herbage allowance did not influence rate of bitingand steers averaged 21 bites min-' . Herbage DMintake per bite was 10 g at the two lowestallowances and increased to 1 5 g at the highestallowance.

Interactions between stage of growth and herbageallowance

The ingestive behaviour of grazing steers at threeconsecutive stages of regrowth of alfalfa wasmodified by the herbage allowance. Significantinteractions were established for herbage DMintake (leaf, / ' < 0 05; stem, P<Q05\ leaf plusstem, P < 0 0 1 ) and for RDM {P<QO\) and

Ingestive behaviour of grazing cattle 125

Table 4. Characteristics of alfalfa pre- and post-grazing for allowances

Variable

Plot areaStem heightStem heightCanopy heightCanopy heightStem mass (DM)Leaf mass (DM)DM

Unit

m^mmmmmmmm

mgstem" '0 /

Of

Status

Pre-Post-Pre-Post-Post-Post-Post-

I

11-3546323388162252

II24-7

Allowance

II

171549390373196350

1722-5

III

21-154741837220137120

22-1

IV

27-154243236720438820

21-5

LSDoo5

1-31717211746

30-8

Table 5. Main effect means of stage of alfalfa regrowth on intake variables and significanceof linear (L) and residual (Q) terms

Variable

Allowance (DM)UtilizationRDMIntake (DM)Intake (DM)Intake (DM)Intake (DM)Leaf intake (DM)Stem intake (DM)Crazing timeBiting rateBites per sessionIntake per bite (DM)Intake per bite (DM)

Units

kg p lo t" 'Proportionkg h a - 'kg per steer sessionkg (100 kg LW)- ' sess ion- 'kg per steer h —kg (100 kg L W ) - ' h - 'kg per steer sessionkg per steer sessionmin per sessionBites m i n - 'Bitesgm g ( k g L W ) - '

I

4-710-399061-770-651-380-511-130-63

8724-021201-063-95

Stage

II

4-300-5512882-220-821-070-401-290-94

12921-327160-863-20

III

5-060-7013913411-251-640-601-791-61130

16-421321-716-33

L

NS

*

NSNS**

****

NS

Q

NSNSNSNS*

NSNSNSNSNSNSNSNSNS

Table 6. Main effect means of herbage allowance on intake variables and significance of linear (L), quadratic(Q), and residual (C) terms

Variable

Allowance (DM)UtilizationRDMIntake (DM)Intake (DM)Intake (DM)Intake (DM)Leaf intake (DM)Stem intake (DM)Grazing timeBiting rateBites per sessionIntake per bite (DM)Intake per bite (DM)

Units

kg p lo t - 'Proportionkg h a - 'kg per steer sessionkg (100 kg LW) - ' per sessionkg per steer h - 'kg (100 kg L W ) - ' h - 'kg per steer sessionkg per steer sessionmin session-'Bites m i n - 'Bites per sessiongm g ( k g L W ) - '

I

2-830-69732

1-980-841-070-450-981-00113

18-820771-034-3

Herbage allowance

II

4-210-5512032-320-811 130-401-301-0112421-025460-96

3-4

III

5-160-5112942-671-001-480-561-501-18118

21-324891-3451

IV

6-560-4415502-890-981-770-611-841-05106

21-021801-505-2

L

^^

******

P<0\

NSNSNSNS

P<0\NS

Q

NSNSNSNSNSNSNSNSNSNSNSNSNSNS

C

NSNSNSNS*

NSNSNSNSNSNSNSNSNS


Table 7. The effect of grazing session on intake variables and unprotected LSD

Variable

Allowance (DM)Utilization (DM)RDMIntake (DM)Intake (DM)Intake (DM)Intake (DM)Leaf intake (DM)Stem intake (DM)Grazing timeBiting rateBites per sessionIntake per bite (DM)Intake per bite (DM)

Units

kg plot" 'Proportionkg h a - 'kg per steer sessionkg (100 kg LW)-'sessionkg per steer h - 'kg( lOOkgLW)- ' h " 'kg per steer sessionkg per steer sessiontnin per sessionBites min-Bitesgm g ( k g L W ) - '

Session

Morning

4-640-5710912-550941190441 45M l131

20-626471 023-78

Afternoon

4-740-5212982360871-530-571 361 01100

20-51999140521

LSDoo5

015003670018007018007007017

7011164

002081

Table 8. Dry matter intake, RDM, utilization and canopy height following grazing forallowances at all stages of growth and unprotected LSD

Stage

IIIIIIIIIIIIIIIIIIIIIIII

LSDo.05

Allowance

IIIIIIIV1IIIIIIVIIIIIIIV

Intake(kg per session)

1 311 661 802291 972022-552342643-27367403039

Utilization

0490400320330-74053054039085070067059009

RDM(kg h a - ' )

750881999992752

135213151733695

137515681926242

Canopy height(mm)

17018520621517021419620214518820219634

herbage utilization (P < 0-05). Since some of thesevariables are related, only data for DM intake persteer session along with RDM, utilization andcanopy surface height are presented (Table 8).

Intake was clearly limited in steers grazingalfalfa at the 25-27-d stage. Steers grazing theseswards at the lowest allowance ingested 1-3 kgDM per session at a rate of 10 kg h-' andconsumed about 49% of the available herbageDM above 5 cm (Table 8) but stopped grazingafter 87 min (Table 5). When grazing the 46-48-d-old swards at the lowest allowance steers ate at1-3 kg h-', grazed for 43 min longer, ingested2-6 kg DM per session and utilized 85% of foragemass above 5 cm. Steers grazing at the mostgenerous allowance ingested 2 29 kg DM persession (2-1 kg h-') when on the youngest swardand 4-03 kg DM per session (19 kg h"') when onthe oldest sward. Utilization was 0-33 and 0 59,respectively.

Grazing behaviour of steers at the intermediatestage of growth was somewhat different fromtheir behaviour at the earlier and later stages ofgrowth and this may have contributed to thesignificance of the interactions (Tables 5 and 8).The overall effect of allowance on rate of intakeand its components was much less and maximumintakes were recorded on allowance III (Table 8).These interactions may have been caused by oneor more of several factors such as under-esti-mation of allowances, improper adjustment oftether lengths and water-induced shrinkage oftether ropes (chains were used as tethers insubsequent experiments). It is also evident thatthe weather was cooler during the intermediatestage as indicated by temperature data (Table 2).

Linear components of interactions betweenstage of growth and herbage allowance weresignificant and probably arose because of increas-ing proportions of herbage mass and herbage


allowance above 20 cm. Animals grazed alfalfa in10-15 cm horizons down to canopy surfaceheights of around 20 cm and then stopped, exceptwhere allowances were severely limiting whenthey grazed canopies down to 15-17 cm (Table 8).As the alfalfa grew and accumulated herbagemass above 15-20 cm, animals were able to utilizeas much as 85% of the allowance. The compo-sition of sward residues were similar acrossallowances but varied with stage of growth. StemDM accounted for 70% of RDM of the youngestsward and 90% of the RDM of the other swards(Table 3).

Grazing session

Ingestive behaviour only differed slightly betweenmorning and afternoon grazing sessions (Table 7)and interactions between this variable, stage ofgrowth and herbage allowance for measuredvariables were few and unimportant. Steers con-sumed 2-6 and 2-4 kg of herbage DM during themorning and afternoon grazing sessions, respect-ively, and these differences were reflected invalues of RDM and utilization. Grazing sessionswere 30 min longer in the morning than in theafternoon and, as rates of biting were similar forboth sessions (20 bites min"'), steers took about600 more bites in the morning. Herbage DMintake per bite was 1 -0 and 1 -4 g for morning andafternoon grazing sessions, respectively. As aconsequence of these differences, herbage DMintake rates in the afternoon (1 • 5 kg h - ' ) exceededthose recorded in the morning (1-2 kg h"') .Differences between swards following grazingwere small. Canopy surface heights were 17 and21 cm following morning and afternoon grazingsessions, respectively, and RDMs were 1091 and1298 kg ha- ' for the same sessions (Table 7).

Discussion

Allowance-availability relationships

Changes in herbage mass and canopy structureassociated with the development of the alfalfaand increasing herbage allowances had similarand additive effects on ingestive behaviour ofgrazing steers (Table 8). Increasing DM allow-ances of alfalfa to 2 5 kg (100 kg LW)-' resultedin a linear increase in the amount and rate of DMingested per session. Reardon (1980) found thatthe LW gain of beef cattle did not respond to

increases in herbage DM allowances of alfalfaabove 3 kg (100 kg LW)- ' daily and it can beinferred that intake followed similar trends.Intake (kg (100 kg LW)-') in our study did notincrease when DM allowances exceeded 4 kg (100kg LW)-' daily (Table 6). Reardon's data reflectgain-allowance relationships established overmany days, whereas the intake-allowance func-tions reported for this experiment were derivedfrom grazing periods of about 2 h. When animalsgrazed alfalfa at stage III, DM intake reached2 75 kg (100 kg LW)- ' daily at DM allowances of4 0 kg (100 kg LW)-' daily with utilization of 0-67(Table 8). Alder and Minson (1963) also useddifference methods to estimate alfalfa intake bysteers strip-grazing alfalfa and estimated organicmatter intakes of 2 5 kg (100 kg LW)-' daily. Thisis approximately equivalent to 2 7 kg DM (100 kgLW)-' daily. Joyce and Brunswick (1977) foundthat when utilization was less than 0 7, LW gainsof beef cattle of 1 kg d- ' were possible. The firstspring crop of alfalfa is usually the tallest andheaviest crop of the year and as the seasonprogresses regrowth crops develop at a faster rateand reach an appropriate stage for grazing atsmaller herbage masses and at lower canopysurface heights (Douglas, 1986). Consequently,maintenance of productivity of beef yearlingsrequires larger herbage allowances and poorerutilization as the season progresses (Joyce andBrunswick, 1977).

Herbage ingested appeared to be largelylimited to the herbage mass in the canopy above20 cm except at the lowest allowance when steersgrazed the canopy down to 15-17 cm. Theproportion of herbage DM mass in the grazedhorizon increased as the crop developed, as didthe proportion of the DM allowance (> 5 cm) inthe grazed horizon. Hodgson (1981), Barthramand Grant (1984) and Forbes and Hodgson(1985a) have noted the sensitivity of grazingbehaviour to the height of the surface horizon andhave observed that animals preferred leafy hori-zons and rejected horizons where stems predomi-nated. It appears that the height at the base of thegrazed horizon was more important than theproportions of leaf and stem in determiningingestive behaviour of animals grazing alfalfa. Inthis study leaf tissue DM accounted for 64% ofthe 1-77 kg ingested by steers grazing alfalfa atstage 1 and 52% of the 3 41 kg ingested at stage 3(Table 5). Leaf DM was 49% of the 1 98 kgingested at the lowest allowance and 64% of the


2 89 kg eaten by steers with the highest allowance(Table 6). Our data show that the leaf DMfraction of rejected (ungrazed) herbage decreasedfrom 0 3 to 01 across stages of growth andincreased from 0-1 to 0-2 with increasing herbageallowance. Steers devoured alfalfa canopies byhorizons of 10-15 cm and, since the proportion ofleaf DM declines progressively down the canopybecause of growth form (Alder and Minson,1963) and progressive acropetal senescence ofleaves, selection of leaf over stem tissue did notappear to be a conscious act of grazing. This is inagreement with the conclusions of Hodgson(1981; 1982b) and Forbes and Hodgson (1985a)regarding diet selection of grazing animals. Thejunction between grazed and ungrazed horizonsof the alfalfa canopy may well be the point on thestem that fractures most readily during theprehension process. In vegetative grasses thisjunction is the surface represented by the top ofthe pseudostems (Barthram and Grant, 1984;L'Huillier et al., 1984); however, there appears tobe no such well-defined junction in grazed alfalfa.

Grazing session and fasting times

Afternoon sessions were about 30 min shorterthan morning sessions and this probablyaccounted for most of the observed differences ingrazing behaviour exhibited by steers (Table 7).Ingestive behaviour of grazing sessions may havebeen modified by the length of the preceding fasts.Grazing in the morning followed overnight fastsof about 16 h while afternoon sessions werepreceded by fasts of about 4 h. According tocurrent theories of intake regulation, eating com-mences because of hunger and ceases because ofsatiety (Baile and McLaughlin, 1987). The rate ofintake is maximal at the start of eating, when theeating drive is unchecked, and declines progres-sively throughout eating sessions as hunger isalleviated and satiety regulating mechanisms areactivated (Dougherty et al., 1987). The afternoongrazing session may have been terminated by theearlier induction and activation of the satietyregulating mechanisms and, if the pattern ofintake followed a decay function, this wouldmean faster rates of biting, larger bites and higherrates of intake.

When herbage intake was expressed on a wetweight basis steers ingested 110 and 11-7 kg ofalfalfa during morning and afternoon grazingsessions, respectively. Since the initial volumes of

ingesta were similar, distension of the gastro-intestinal tract by volume of fill cannot bediscounted as the mechanism that stopped graz-ing (Waldo, 1986). Thomson et al., (1985),however, concluded from their study with sheepthat grazing animals did not regulate grazingpatterns with reference to rumen fill. They alsoobserved that maximum rumen fill was onlyachieved at the end of the afternoon grazingperiod and commented that it might have contri-buted to its termination. Ambient temperaturesduring the afternoon were close to, or slightlyabove, the thermoneutral zone of cattle and alongwith higher radiation loads may have also contri-buted to the early cessation of afternoon grazingthrough a thermostatic mechanism (Weston,1981). Swards made less acceptable by trampling(Brown and Evans, 1973) and fouling (Forbesand Hodgson, 1985b) may have also contributedto decreased grazing times, particularly if theeating drive in the afternoon was weakened bysatiety. It is apparent that the length of fastbetween grazing sessions can have considerableimpact on ingestive behaviour.

Many researchers or reviewers have notaccounted for or have omitted consideration ofthe effects of fasting time and, hence, hunger-satiety status of the ingestive behaviour of grazinganimals. Researchers have the option of control-ling fasting times and grazing periods, as we did inthis study and as Phillips and Leaver (1986) did intheir investigation of forage supplementation ofgrazing dairy cows, to reduce confounding effectsof uncontrolled fasting times on hunger-satietyrelationships and minimize the effects of weatherand social behaviour on ingestive processes. Free-ranging beef cattle have two major grazing ses-sions each day in summer at this location, onecommencing at dawn and the other at dusk withan average overnight fast of 9 h (Wahab, 1984).Since estimates of daily DM intake were similarto those reported by Alder and Minson (1963) itappears likely that the use of two grazing periodseach day and an overnight fast was not undulyrestrictive in terms of DM intake per day.

Ingestive behaviour

Average rates of DM intake per animal reached2 kg h - ' when alfalfa canopies and herbageallowances were favourable. Rates of this orderhave been recorded for heifers grazing alfalfaunder similar conditions (Dougherty et al., 1987).


These rates of intake were associated with inges-tion of high proportions of leaf tissue rather thanstem tissue. Preference, conscious or otherwise, ofgrazing livestock for leaf tissue has been wellestablished (Alder and Minson, 1963; O'Connor,1967; Hodgson, 1981; Allen et al., 1986a; b). Withfavourable swards and adequate allowancessteers were able to form bites as high as 2 g ofherbage DM per bite and sustain high rates ofDM intake. Bites of similar size have beenrecorded for adult cows (480 kg LW) grazing highyielding ryegrass (Lolium perenne L.) pastures(Hodgson and Jamieson, 1981) and were similarto those we recorded for heifers (370 kg LW)grazing alfalfa (Dougherty et al., 1987). Herbageintake per bite depends on the density of herbagemass in the grazing horizon, the herbage-gathering capacity of the mouth and tongue(Forbes et al., 1985) and the intensity of the eatingdrive as expressed by the hunger-satiety controlmechanisms (Baile and McLaughlin, 1987). Inour experiment the overnight fast ensured, atleast for morning sessions, that the desire to grazewas not tempered at the onset by satiety regu-lating mechanisms and that the estimates ofherbage DM intake per bite and rate of bitingwere largely moderated by sward properties. Oursteers formed these large bites at a slow rate of16 bites min"'. Inverse relationships betweenherbage intake per bite and rates of biting arewell-established (Hodgson and Jamieson, 1981).It has also been established (Hodgson, 1981), andit is also evident from our data, that the grazinganimal cannot sustain high rates of intake byacceleration of rate of biting to compensate forthe ingestion of small amounts of herbage intakeper bite.

Daily DM intake of steers reached 8 kg d"' atthe most generous allowance of the alfalfa 46-48days old. Steers of the frame size and LW (270 kg)used in this study should ingest about 6 4 kg d"'of alfalfa DM according to the NationalResearch Council equations (National ResearchCouncil, 1984). These estimates of DM intakeappear to be too low to support the daily LW gainof growing beef cattle on alfalfa pastures reportedin the literature (Douglas, 1986). The net energyfor maintenance (NEm) of the ingesta of animalsin the present experiment may have been higherthan the concentrations of NEm used to estimateintake since animals did not consume much of thebasal stem material. The small particle size oflegume ingesta, related to the dimensions of the

trifoliates (Troelsen and Campbell, 1968), thesmall proportion of cell wall in the DM and theirprompt disintegration in the rumen are thoughtto contribute to the rapid disappearance oflegumes from the reticulo-rumen and reduce theircontribution to gut-fill per unit of DM intake(Waldo, 1981). As a result consumption of leafylegumes may exceed estimates made usingNational Research Council (1984) equations.

In this experiment DM intake increasedlinearly with herbage DM mass (> 5 cm) up to4656 kg ha- ' . Herbage intake by grazing cattleincreases with herbage DM mass of temperateswards with the asymptote being poorly definedand in the range of 1100-2800 kg ha- ' (Hodgson,1977). There are major structural differencesbetween an alfalfa canopy (Alder and Minson,1963; Allen et al., 1986a; b) and a temperategrass-clover pasture (Hodgson, 1981), this isrefiected in the rigid grazing management recom-mendations for alfalfa (Douglas, 1986). Sincerotational grazing of alfalfa is essential for itsproductivity and longevity, and grazing manage-ment does not advocate close grazing or practicesthat damage crowns, crown buds or crownshoots, measurement of alfalfa herbage mass tothe soil level is not warranted, nor is it practical inthis location for crowns are slightly elevatedabove the soil surface by frost-heave and loca-lized soil erosion.

Conclusions

Grazing of alfalfa by beef cattle followed well-established trends in terms of the effects on swardstructure on ingestive behaviour. Herbage intakewas very responsive to changes in herbage massand allowance. Grazing was essentially limited toa canopy horizon whose baseline was about 20 cmabove the soil surface. Marked selectivity of leafover stem at high rates of intake was apparentlynot an act of conscious selectivity but a con-sequence of canopy structure. The regulation ofgrazing sessions and fasting times, together withthe statistical efficiency of the design allowed bythe use of tethered cattle, has many advantages ininvestigations of plant-herbivore relationships.

The pattern of grazing of alfalfa at highherbage mass and with a generous allowance peranimal, under rotational grazing and in theabsence of conscious selectivity, favours the useof a two-phase grazing sequence as advocated by


Blaser (1982) in which top grazers (animals withhigh requirements in terms of intake and quality)precede bottom grazers (animals with lowerrequirements).

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(Received 21 October 1986; revised 8 August 1987)

ingestive behaviour of beef cattle grazing alfalfa (medicago sativa l.)+

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