Metabolizable protein (MP) supply and amino acid balance were manipulated through selection of highly digestible rumen-undegradable protein (RUP) sources. Effects on production efficiency and N utilization of early post-partum dairy cows were determined. Forty-two multiparous and 16 primiparous Holstein cows were assigned to the diets in a randomized complete block design immediately after parturition with 3-wk experimental periods, and then were fed a ration for 120 days in milk. Diets were formulated to provide 3 concentrations of dietary RUP (LRUP 6.65, MRUP 7.72 and HRUP 8.79% of dry matter (DM)) while rumen-degradable protein remained constant (11.3% of DM). Diets contained 26.30% alfalfa hay, 12.60% corn silage, 9.50% sugar beet pulp and 51.5% concentrate in DM basis. Ingredients within the diets were equal across treatments except for fish meal and corn gluten meal that partially replaced with steam rolled barley and soybean meal. Dry matter intake linearly increased by the treatments. Milk yield, Fat corrected milk (FCM) and protein content and yield increased significantly when cows were fed the diets with greater RUP, but milk fat and lactose was not different between treatments. Body weight (BW) changes was improved with intake of high RUP but Body condition score (BCS) changes had significant difference and improved by increasing RUP in the diet. The efficiency of N use increased linearly. Milk urea N and predicted urinary N increased linearly when cows were fed higher amounts of RUP, but differences between the control treatment and high RUP diets were not significant. Fecal N and N balance did not have significant difference. Total tract digestibility of DM and crude protein (CP) intakes increased significantly with greater RUP. In general, increasing amounts of MP and RUP improved productive performance and BCS status of fresh cows and enhanced digestibility of DM and CP. تفاصيل المقالة