Response of early lactation cows to ruminally undegradable protein in the diet

The objective of this study was to determine the response of dairy cows in early lactation to diets based on the recommendations of the Agricultural Research Council or the NRC for sustaining milk yield. Diets were formulated to satisfy the nutrient requirements of Holstein cows weighing 600 kg and yielding 35 kg of 3.5% fat milk/d according to either Agricultural Research Council recommendations or NRC recommendations for RUP. A third diet was a 1:1 (wt/wt) mixture of the Agricultural Research Council and the NRC diets. The same forage was fed in all diets at a forage to concentrate ratio of 40:60, and the RUP supply was altered by substituting fish and corn gluten meal for canola meal. The calculated effective degradabilities of CP for the TMR were 70.1, 66.1, and 62.1% for the Agricultural Research Council diet, the mixture of Agricultural Research Council and NRC diets, and the NRC diet, respectively. Milk composition was similar for the three dietary treatments. Multiparous cows showed a linear yield response (30.4, 31.6, and 33.7 kg/d) to increasing inclusion of the NRC concentrate in the diet. No response to additional RUP was observed for first lactation heifers. Agricultural Research Council recommendations for dietary RUP underestimated the requirements for multiparous cows in early lactation yielding > 25 kg of milk/d.