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1 Department of Clinical Studies, Center for Animal Health and Productivity, School of Veterinary Medicine, University of Pennsylvania, Kennett Square 19348
Four multiparous Holstein cows were used in a 4 x 4 Latin square to investigate the effects of protein concentration, degradability, and quality on plasma urea concentration and milk N constituents. Diets varied in the amount and proportion of RDP and RUP relative to NRC requirements: diet 1, excessive RDP, deficient RUP; diets 2 and 3, balanced for 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 Carbohydrate and Protein System. Diets contained 34% corn silage, 19% alfalfa haylage, and 49% concentrate @M basis). Concentrates varied in amounts of urea and soybean, corn gluten, and fish and blood meals.
Concentrations of urea N and NPN in milk varied among diets: diet 1, 19 and 34 mg/dl; diet 2, 16 and 31 mg/dl; diet 3, 15 and 30 mg/dl; and diet 4, 23 and 39 mg/dl, respectively. Increases in NPN concentration were attributed to increases in the urea fraction of NPN. Intake of RUP and AA balance influenced milk true protein content: diet 1, 2.89%; diet 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 type, and protein quality.
Key Words: protein • urea nitrogen • nonprotein nitrogen • milk
Submitted on October 14, 1994
Accepted on June 16, 1995
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