RESPONSES IN UREA AND TRUE PROTEIN OF MILK TO DIFFERENT PROTEIN FEEDING SCHEMES FOR DAIRY-COWS

Four multiparous Holstein cows were used in a 4 x 4 Latin square to in vestigate the effects of protein concentration, degradability, and qua lity on plasma urea concentration and milk N constituents. Diets varie d in the amount and proportion of RDP and RUP relative to NRC requirem ents: diet 1, excessive RDP, deficient RUP; diets 2 and 3, balanced fo r RDP and RUP; and diet 4, excessive RDP, balanced for RUP. Diet 3 was formulated for optimal AA balance as predicted by the Cornell Net Car bohydrate and Protein System. Diets contained 34% corn silage, 19% alf alfa haylage, and 49% concentrate (DM basis). Concentrates varied in a mounts of urea and soybean, corn gluten, and fish and blood meals. Con centrations of urea N and NPN in milk varied among diets: diet 1, 19 a nd 34 mg/dl; diet 2, 16 and 31 mg/dl; diet 3, 15 and 30 mg/dl; and die t 4, 23 and 39 mg/dl, respectively. Increases in NPN concentration wer e attributed to increases in the urea fraction of NPN. Intake of RUP a nd AA balance influenced milk true protein content: diet 1, 2.89%; die t 2, 2.90%; diet 3, 3.01%; and diet 4, 2.95%. The proportions of true protein and urea in milk are influenced by CP concentration, protein t ype, and protein quality.