RESPONSE OF EARLY LACTATION COWS TO RUMINALLY UNDEGRADABLE PROTEIN INTHE DIET

The objective of this study was to determine the response of dairy cow s in early lactation to diets based on the recommendations of the Agri cultural 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 eit her Agricultural Research Council recommendations or NRC recommendatio ns 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 di ets 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 o f Agricultural Research Council and NRC diets, and the NRC diet, respe ctively. 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 underest imated the requirements for multiparous cows in early lactation yieldi ng >25 kg of milk/d.