Effects of Amounts and Degradability of Dietary Protein on Lactation, Nitrogen Utilization, and Excretion in Early Lactation Holstein Cows

Five treatment diets varying in crude protein (CP) and rumen undegradable protein (RUP) were calculated to supply a postruminal lysine to methionine ratio of about 3:1. Diets were fed as a total mixed ration to 65 Holstein cows that were either primiparous (n = 28) or multiparous (n = 37) from 21 to 120 d in milk to determine effects on lactation and nitrogen utilization. Crude protein % and calculated RUP (% of CP) of diets on a dry matter (DM) basis were: 1) 19.4, 40 (HPMU), 2) 16.5, 34 (LPLU), 3) 16.8, 40 (LPMU), 4) 16.8, 46 (LPHU), 5) 17.2, 43 (LPHU + UREA), which is the result of adding 0.4% of the diet DM as urea to LPHU. The corn silage-based treatment diets contained an average of 24% acid detergent fiber and 1.6 Mcal/kg net energy of lactation. Milk urea nitrogen (MUN) concentrations and body weights (BW) were used to calculate predicted amounts of urinary nitrogen (N) using the relationship: urinary N (g/d) = 0.0259 x BW (kg) x MUN (mg/dl). Cows fed HPMU had greater CP and RUP intakes, which resulted in higher concentrations of plasma urea nitrogen, rumen ammonia, MUN, and predicted urinary N. Milk yield, fat yield, fat percent, protein yield, and protein percent were not significantly different among treatments. Parity primarily affected parameters that were related to body size and not measurements of N utilization. The interaction of treatment and parity was not significant for any measurements taken. In this study, cows fed LPHU had significantly lower MUN and predicted urinary N without limiting production. These results demonstrate the potential to optimize milk production while minimizing N excretion in lactating dairy cattle.