effect of pasture allowance and energy supplementation upon dairy cows milk yield

Introduction

Milk production in Argentina is based on grazing perennial and annual pastures of temperate climate. Concentrates are used as supplements to balance diets, cope with seasonal shortages and higher requirements of high genetic merit animals, especially during certain stages of their productive cycle. The milk sector must get high yields and low prices in order to compete with the expansion of soybean continuous cropping

(INTA, 2003). Therefore, pasture management practices that allow achieving high intake and highly efficient utilization of forage must be achieved. Furthermore, efficient criteria of supplementation that optimizes the use of concentrates (Mayne and Peyraud, 1996, cited by Delaby et al., 2001) must also be accomplished.

The mechanisms that regulate pasture intake are complex (Ingvartsen and Andersen, 2000), because of nutritional and non-nutritional factors interactions (Hogdson, 1985). According to Holmes (1987), some factors that determine intake may be modified through management, for example the amount of forage available per

Effect of pasture allowance and energy supplementation upon dairy cows milk yield

Hugo J. Alvarez1, Luciana Dichio1, Mariela A. Pece1, Carlos A. Cangiano2 and Julio R. Galli1

1Facultad de Ciencias Agrarias, Universidad Nacional de Rosario2Instituto Nacional de Tecnología Agropecuaria EEA, Balcarce

C.C. 14 S2125 ZAA Zavalla, Argentina

Abstract

H.J. Alvarez, L. Dichio, M.A. Pece, C.A. Cangiano and J.R. Galli. 2006. Effect of pasture allowance and energy supplementation upon dairy cows milk yield. Cien. Inv. Agr. (In English) 33(2):81-88. The effect of different levels of daily allowance and supplementation on intake, milk production and composition and per hectare productivity was studied. Pasture consisted of an association of alfalfa (Medicago sativa), tall fescue (Festuca arundinacea) and prairie grass (Bromus wildenowii) and high moisture corn was used as supplement. 32 cows were allocated to four levels of pasture allowance (PA) and two levels of energy supplementation (ES). Pasture intake and individual and per hectare production were affected by PA, but ES had no significant effect. No interactions were detected between any of the variables. Milk composition were not affected by PA, whereas only non fat solids were affected by ES. Liveweight change and condition score did not show differences among treatments. Urea level in both, plasma and milk, was not affected by PA, whereas the level of urea in plasma showed a trend to be different and the level of urea in milk was significantly different for ES. Critical levels of pasture allowance which directly affect the productivity per hectare, need to be reached in order to perform a significant increase of individual production. Milk fat and protein concentration did not appear to be responsive variables to different levels of supplementation and allowance, whereas the little variation in urea levels, as a response to a greater level of supplementation, did not make this a valuable tool to improve the milk protein constituents.

Key words: Daily allowance, dairy cow, energy supplementation, intake, milk production and composition.

Received 01 Jun 2005; Accepted 22 December 2005.1 Corresponding author: [email protected]

RESEARCH PAPER

Cien. Inv. Agr. 33(2): 81-88 2006www.rcia.puc.cl

82 CIENCIA E INVESTIGACION AGRARIA

animal, since the level of forage allowance may affect intake, individual yield and yield per hectare.

Energy is the main limiting nutrient in grazing systems, especially if they include alfalfa. Therefore, energy concentrates are usually included in nutritional strategies of high yielding dairy cows. Substitution effect would take place regarding pasture intake, which will be essentially influenced by the daily forage allowance (Bargo et al., 2002).

Supplementation effect on milk production will be best achieved under pasture restriction (Bargo et al., 2002), even more, lower yielding animals might show no response when pasture is offered ad-libitum (Kellaway and Porta, 1993).

Milk constituents, mainly protein, may be affected by quality and amount of supplementation. Temperate pastures tends to have high crude protein content and low levels of non-structural, soluble carbohydrates (Elizalde and Santini, 1992; Beever, 1993). Furthermore, forage protein is highly degradable in the rumen, which might affect the protein-energy synchronism, generating a rumen environment with high concentrations of ammonia nitrogen (N-NH3), leading to inefficient utilization of dietary protein (Beever, 1993). Excess N-NH3 is metabolized in the liver contributing to increase blood urea. At the same time, the mammary gland is one of the excretion routes for blood urea, there is a high correlation between plasma urea and the amount of urea in milk (r=0,89; DePeters and Ferguson, 1992). High levels of urea in milk are possible to be found in production systems like the ones described above, thus altering its composition. One of the alternatives to improve efficiently the use of dietary nitrogen is to have more energy available at the ruminal level which may be achieved by using highly degradable grains in the rumen, like high moisture corn (Rearte et al., 1997; Alvarez et al., 2001).

Decision making regarding the amount of forage that should be offered to animals, as well as the quality and amount of energy to be supplemented, is a common practice in dairy

farms where rotational pasture systems are used. Hence, the objective of this study was to evaluate the effect of different amounts of pasture allowance and supplementation upon intake, milk production, milk composition and productivity per hectare.

Materials and methods

The study was carried out at the experimental dairy farm “J. F. Villarino” of the School of Agricultural Sciences, Universidad Nacional de Rosario, Zavalla, province of Santa Fe, Argentina. The experimental period took place during spring and lasted 10 weeks from September 15 onwards. A pre-experimental period of two weeks was used.

A mixture of alfalfa (Medicago sativa), tall fescue (Festuca arundinacea) and prairie grass (Bromus wildenowii) was used and high moisture corn grain (HMC) was used as supplement. Chemical composition and digestibility of both resources is shown in Table 1. Alfalfa, tall fescue and prairie grass were established at 12; 6 and 4 kg·ha-1 seed rates, respectively, being the contribution of alfalfa between 50 and 60 % of the total dry matter (DM) offered. 32 Holando Argentino multiparous cows were used with a live weight of 541 ± 55.8 kg and an average lactation period of 97 ± 18 days at the beginning of the trial. The cows were milked twice daily at 6.00 and 16.00 h. They were allocated to a split plot design. Milk production and composition registered during 15 days prior the initiation of the experience was used as covariable (Steel and Torrie, 1980). Four levels of pasture allowance (PA) were established as main plot, and two levels of energy supplement (ES) as secondary plot, with four repetitions. Starting from a potential estimated DM intake (PI) of forage of 17.4 kg·cow-1·day-1 (Galli et al., 1999), the levels of PA were high= 2.3 PI; middle high = 2 PI; middle low = 1.5 PI and low = 1 PI, resulting for each PA in 40; 34.8; 26.1 and 17.4 kg·cow-1·day-1. ES levels were: High = 7 kg DM·cow-1·day-1 and Low = 3.5 kg DM·cow-1·day-1.

A rotational with daily strips grazing system was used; with a resting period of 25 ± 3 days. Double sampling with disc (Spada and Cangiano, 1991)

83VOL 33 N˚2 MAY - AUGUST 2006

methods with commercial reagents (GT Laboratorios, Rosario, Argentina).

Results and discussion

Forage intake, individual production and per hectare production were significantly affected by PA (p < 0.05) and not by ES (p > 0.05) (Table 2). All the concentrate offered was eaten.

Forage intake between high, middle, and low levels of PA showed significant differences, being the greatest between the extreme levels (6 kg DM). This result was similar to the one reported by Stakelum (1986) and Kloster et al. (2000). No significant differences were observed between middle high and middle low PA values.

Highest milk yield was obtained in cows receiving high PA compared to the other three PA (25.3 L·cow-1·day-1 vs. 22 L·cow-1·day-1 for the average middle high, middle low and low PA) was consistent with Robaina et al’s (1998) findings. They also found greater milk yield and more forage intake (and therefore total) with dairy cows receiving higher allowances.

These results are of great importance consider-ing the impact that these practices may have on production systems, especially considering that in dairy farms these decisions are taken at least daily. Considering the two extreme allowances (high and low) it can be concluded that in order to obtain 4 more L of milk, it was necessary to offer 22.6 kg more forage dry matter. In terms of calories, to cover the 4.5 Mcal of additional metabolizable energy that the cows required to produce 4 L more of milk, it was necessary to provide 61 Mcal of metabolizable energy, the equivalent of 55 L of milk.

Hence, it is interesting to evaluate what happened to yield per surface unit, and relate those values to individual production. The two lowest allowances were significantly different between each other and compared to the two highest ones. Low PA reached almost twice the production than high PA (2605 vs 1330 L·ha-1, respectively). High allowance of forage enabled increasing individual production, but decreased the productivity by unit of surface. These results

Component Pasture, High moisture % corn, %

In vitro dry matter digestibility 75.2 ± 0.56 89.8Crude protein1 21.3 ± 1.02 10.4Neutral detergent fiber1 27.5 ± 1.84 --Acid detergent fiber1 15.4 ± 1.55 4.3

Table 1. In vitro dry matter digestibility and chemical composition of association pasture and high moisture corn.

1 Based on dry weight matter. ± Standard deviation.

technique was used to determine the surface for each treatment and it was adjusted weekly. Supplement was offered in individual troughs, in halves during milkings.

Forage intake (kg DM·cow-1·day-1) was estimated every three alternate weeks of the experimental period, using the difference between the amount of forage before and after grazing (Spada and Cangiano, 1991), divided by the number of animals of each plot. Concentrate intake was determined by the difference between daily offered and refused.

Individual milk production was registered daily. Milk composition was measured weekly by composite samples (morning and afternoon milking). Fat, protein, non fat solids (NFS) and total solids were determines using an infrared system (Milkoscan). Total animal milk yield and the surface per treatment was used to calculate per hectare yield.

Live weight change (LWC) was estimated by weighing the animals individually over two consecutive days at the beginning and end of the experimental period, after milking and prior to animal access to water intake. Initialy (ICS) and final (FCS) condition score was measured simultaneously by three evaluators independently using the five point scale (Fattet and Jaurena, 1988).

Jugular vein pre-prandial blood samples were taken weekly to determine urea in plasma (UP). Blood was collected in heparinized tubes (5 units·mL-1 of blood), plasma was obtained by centrifugation (2000 x g during 15 min at room temperature). With the same frequency, composite milk samples were obtained to determine urea in milk (UM), using enzymatic


are congruent with those stated by Holmes (1987); high allowance allows reaching higher levels of intake and high levels of individual yield, however lower stocking rate may decrease forage utilization efficiency (Castro et al., 1993), and also per hectare production (Holmes, 1987). It might be concluded that the level of forage allowance, and stocking rate, had a high impact on the productivity of the established systems, which should be related with a greater efficiency of forage utilization (White, 1987; Kloster et al., 2000).

Intake and milk production did not show, as well as the rest of the variables that were studied, interaction between PA and ES (p>0.05). The amount and quality of the forage offer (Table 1), and taking into account that intake was higher than 11 kg DM·cow-1·day-1, suggests allowance treatments did not show forage restriction in such a way that differences could be observed. Delaby et al. (2001), in a study with high yield dairy cows, working with allowances of 12 to 22 kg DM·cow-1·day-1 and supplementation levels of 0 to 5.4 kg DM·cow-1·day-1, had reported a lineal and added responses in milk production and composition. As in this study, they also found no significant interactions between allowance

and supplementation, stating the hypothesis that perhaps smaller allowances of 10 kg DM·cow-1·day-1 are necessary in order to find different responses.

Supplementation levels did not significantly affect forage intake. However, low ES always showed higher forage intake than high ES, being the highest with the highest PA. This trend was also found by Bargo et al. (2002), who has steated that the larger the amount of forage offered, would increase the substitution rate of forage by concentrate.

Milk production does not show significant differences for different levels of ES, which was consistent with Kellaway and Porta (1993). From the analysis of pasture studies using medium yield cows, no response to supplementation was observed, even negative responses were detected when high quality pasture was offered ad-libitum. Very high quality forage (IVDMD 75.2 %) was used in this study, and although it was not offered ad-libitum, less allowance in turn equates to the potential intake of the animals, which could not be considered an important level of restriction (Meijs, 1981).

Whereas all the aforementioned plus the

Table 2. Pasture intake and individual and per hectare milk yield for different levels of pasture allowance and energetic supplementation (ES).

Pasture Energetic supplementation TotalVariable allowance High Low pasture SE1

allowance

Pasture intake, High 16.6 18.5 17.5 a2

Kg DM·cow-1·day-1 Middle High 12.9 14.8 13.9 b Middle Low 12.3 13.6 13.0 b 1,16 Low 11.1 11.9 11.5 c Total ES 13.2 14.7 13.7

Milk production, High 24.9 25.7 25.3 aL·cow-1·day-1 Middle High 22.4 21.5 22.0 b Middle Low 22.6 22.9 22.7 b 0,88 Low 22.4 20.2 21.3 b Total ES 23.1 22.6 22.8

Per hectare production, High 1307 1353 1330 cL·hectare-1 Middle High 1346 1290 1318 c Middle Low 1850 1878 1864 b 0,88 Low 2740 2471 2605 a Total ES 1810 1747 17791 Standard Error. 2 Between files, means with different letters differ.


relatively low level of average production of the cows, their energy requirements were easily covered and therefore no response to larger amounts of concentrate was observed. This would also explain the lack of significant differences between treatments (p>0.05) for LWC, ICS and FCS. LWC (kg·day-1) was 0.639 ± 0.131, ICS and FCS were 2.67 ± 0.30 and 3.10 ± 025, respectively. At the beginning of the experimental period, partition of nutrients are toward body reserves (Tucker, 1985), which would explain the live weight gains and condition score recovery. Gagliostro et al. (1996) state that when the amount and quality of pasture was adequate, milk production response to supplementation was low, especially in animals of medium and low yield potential. Probably, the extra energy ingested is for rebuilding body reserves.

PA did not alter any values of milk composition (p>0.05). ES only had effect on NFS (p<0.05) (Table 3). Robaina et al. (1998), in a series of trials with various levels of forage allowance and supplement, found differences in milk

production, but not in its composition, except for those cases in which the animals that were supplemented improved protein content as compared to controls that did not receive supplements. Gallardo et al. (1991) have reported no differences in the chemical composition of milk when grazing cows were supplemented with different amounts of corn grain, contrary to what is generally reported in the literature . This type of contradiction is perhaps what leads Sauvant and Van Milgen (1995) to carry out a debate to review some classical nutritional approaches that do not account for lower responses in production, than those expected in theory.

Although UP and UM levels did not differ significantly (p>0.05) for PA, the same did not happen for ES (Table 4). UP tended to be less (p=0.12) and UM was significantly less (p<0.01) for high level ES. The correlation coefficient between UM and UP was 0.66 (p<0.05).

Plasma and milk urea values were high compared to the those reported in the

Pasture Energetic supplementation TotalVariable allowance High Low Pasture SE1

allowance

Fat High 3.16 3.41 3.28 Middle High 3.33 3.23 3.28 Middle Low 3.54 3.23 3.38 0.16 Low 3.25 3.22 3.24 Total ES 3.32 3.27 3.30

Protein High 3.12 3.04 3.08 Middle High 3.10 3.03 3.07 Middle Low 3.00 2.94 2.97 0.07 Low 3.03 2.97 3.00 Total ES 3.06 3.00 3.03

Non fat solids High 8.52 8.39 8.45 Middle High 8.52 8.39 8.45 Middle Low 8.44 7.73 8.08 0.34 Low 8.39 8.21 8.29 Total ES 8.47 a2 8.18 b 8.32

Total solids High 11.21 11.81 11.51 Middle High 11.78 11.17 11.47 Middle Low 11.91 11.46 11.68 0.19 Low 11.64 11.40 11.52 Total ES 11.63 11.46 11.541Standard Error.2Between rows, means with different letters differ.

Table 3. Milk composition (%) for different levels of pasture allowance and energetic supplementation (ES).


international literature (13.4 and 14.21 mg·dl-1 for plasma and milk urea, respectively, DePeters and Ferguson, 1992), but consistent with those that tend to happen in animals grazing alfalfa (Castillo, 2002). The lower levels of urea -especially in milk- found in cows that received the highest levels of ES, have a basis considering research by Castillo and Gallardo (1995), who stated that there was a close relationship between energy supplementation of animals in pasture and non protein nitrogen in milk (R2=0.63). Hermansen et al. (1994), also under the same conditions, measured lower levels of uremia by replacing a concentrate rich in fat with one rich in carbohydrates that contributed energy in rumen. Corbellini (1995) established that milk urea levels higher than 30 mg·dl-1, and milk protein percentages lower than 3.2 %, indicate energy deficit and excess protein in the diet. If so, this experience performed in pastures with 21 % of crude protein, supplementation with 7 kg of DM of highly degradable ruminal grain, should produce more important reduction of urea levels than the ones registered, considering that these always were above 30 mg·dl-1. Although the differences were significant, the decreased level of milk urea attained because of the high level of supplementation (-3.22 mg·dl-1) barely had productive relevance. These results also support the need of reconsidering certain nutritional approaches (Sauvant and Van Milgen, 1995).

Under the conditions of this study, milk production responded to changes in forage

allowance but not to changes in the amount of energy supplementation. This would make it necessary to revise the amounts of supplements usually used in high quality temperate pastures. However, to achieve a considerable increase in individual production, it is necessary to offer more than twice the potential intake, which considerably hinders per hectare production in systems that include associated alfalfa and grass pastures as nutritional base. Milk fat and protein concentration were not sensitive variables to different levels of supplementation and forage allowance. Regarding blood and milk urea levels, responses attained with larger levels of supplementation did not make this a valuable tool to improve milk protein.

Resumen

Se estudió el efecto que distintos niveles de asignación diaria y suplementación producen sobre el consumo, la producción y composición de la leche y la productividad por hectárea. La pastura utilizada fue una asociación de alfalfa (Medicago sativa), festuca (Festuca arundinacea) y cebadilla criolla (Bromus wildenowii) y el suplemento fue grano de maíz húmedo. Se utilizaron 32 vacas y se establecieron cuatro niveles de asignación de pastura (PA) y dos niveles de suplementación energética (ES). El consumo de pastura y la producción individual y por hectárea fueron afectados por PA y no por ES. Al igual que para el resto de las variables, no se detectaron interacciones entre PA y ES. La composición de

Pasture Energetic supplementation TotalVariable allowance High Low Pasture SE1

allowance

High 36.55 41.25 38.90 Urea in plasma, Middle High 37.65 41.90 39.78 mg·dl-1 Middle Low 34.35 38.60 36.48 2.35 Low 39.97 37.75 38.86 Total ES 37.13 39.88 38.51

High 35.55 38.23 36.89 Urea in milk, Middle High 32.70 39.35 36.03 mg·dl-1 Middle Low 33.10 36.03 34.56 1.75 Low 35.53 36.18 35.85 Total ES 34.22 b2 37.44 a 35.83

Table 4. Urea in plasma and urea in milk for different levels of pasture allowance and energetic supplementation (ES).

1 Standard Error.2 Between rows, means with different letters differ.


la leche no fue afectada por PA, mientras que ES sólo tuvo efecto sobre los sólidos no grasos. La evolución del peso vivo y del estado corporal no mostraron diferencias entre tratamientos. Los niveles de urea en plasma y leche no difirieron significativamente para PA, no ocurriendo lo mismo para ES, donde el nivel de urea en plasma tendió a diferir y el nivel de urea en leche difirió significativamente. Se concluyó que para conseguir aumentos significativos en la producción individual es necesario ofrecer niveles de asignación que afectan negativamente la producción por hectárea. La concentración de grasa y proteína de la leche no parecieron ser variables sensibles a distintos niveles de suplementación y asignación, mientras que la escasa variación en los niveles de urea debida al mayor nivel de suplementación relativizó el valor de esta herramienta para mejorar la composición de la proteína láctea.

Palabras clave: Asignación diaria, consumo, producción y composición de leche, suplementación energética, vaca lechera.

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