Use of the Cornell net carbohydrate and protein system and rumen-protected lysine and methionine to reduce nitrogen excretion from lactating dairy cows.

A study was carried out to determine whether the addition of rumen-protected Lys and Met to ration formulations allowed a reduction in dietary crude protein (CP) without jeopardizing total milk or milk protein yields. Eighteen multiparous Holstein cows were randomly assigned to treatment sequences in a replicated 3 x 3 Latin square design. Total mixed rations were balanced according to degradation and rates of passage of protein and carbohydrates using the Cornell Net Carbohydrate and Protein System. Rations differed in percentages of CP (18.3, 16.7, and 15.3% for rations 1, 2, and 3, respectively), but energy was held constant. Rations 2 and 3 were supplemented with rumen-protected Lys and Met. Milk, blood, and rumen fluid samples were taken during the 2nd and 3rd wk of each 28-d experimental period. Total collection of urine and feces occurred during the last 5 d of each experimental period. Cows fed ration 1 had a higher milk yield (34.2 vs. 32.8 kg/d) and DMI than did cows fed rations 2 or 3, but milk protein output was not different among groups. Nitrogen efficiency, milk N as a percentage of intake N, improved as percentages of CP in the rations were reduced. Blood urea N values were 15.9, 12.9, and 10.0 mg/dl for cows fed rations 1, 2, and 3, respectively. Apparent digestibilities of CP and urinary N excretion decreased as the percentages of CP in the rations decreased. Results indicated that it is possible to make more efficient use of CP by using rumen-protected amino acids. This procedure may result in less than maximum milk yield, but milk protein output can be maintained.