Lactational responses to ruminally undegradable protein by dairy cows fed diets based on alfalfa silage

Lactational responses to protein supplementation of diets containing 60% of DM as alfalfa silage were evaluated. Sixty muliparous Holstein cows were fed a covariant diet for the first 3 wk postpartum, blocked by calving date, and randomly assigned for 14 wk to one of six isonitrogenous (19.4% CP) diets. Diets were formulated with soybean meal, a blend of animal by-products, or both, and contained 5.0, 5.6, 5.6, 6.2, 6.1, and 6.8% ruminally undegradable protein (DM basis). Percentage of ruminally undegradable protein or source of supplemental protein did not affect 3.5% FCM (39.4 kg/d), milk fat yield (1.38 kg/d), milk protein percentage (2.83%), milk urea (7.66 mM), or plasma urea (8.91 mM). However, cows fed diets supplemented with soybean meal had higher DMI (26.2 vs. 24.7 kg/d), milk yields (40.4 vs. 39.1 kg/d), and milk protein (1.15 vs. 1.09 kg/d) yields, but lower milk fat concentration (3.42 vs. 3.53%) and body condition score (2.85 vs. 2.93) than cows fed diets containing a blend of animal by-products. The lack of response to ruminally undegradable protein was partially caused by higher than predicted DMI (5 to 15% above NRC predictions); all diets provided at least 1.3 kg of ruminally undegradable protein, and there was no beneficial effect from ruminally undegradable protein intake increases to 1.6 kg/d.