The effect of protein degradability on milk composition and production of early lactation, somatotropin-injected cows.

Twenty multiparous Holstein cows in early lactation that received 500 mg bST injected every 2 wk were assigned to one of two treatments to examine the influence of diets that varied in degradability of protein. Effects of degradability were determined on milk production and components and on nutrient digestibility. Treatments consisted of a basal ration (control) containing soybean meal as its primary degradable protein source and a ration (treatment) containing corn gluten and meat and bone meals as the primary undegradable protein source, representing 33% undegradable protein in CP. The undegradability of protein sources did not influence DMI and BW. Milk yield, 3.5% FCM, and production efficiency of bST-treated cows were not affected by increased undegradable protein in the diet. Milk fat and SNF were not significantly increased by treatment. Lactose was significantly higher for the control diet (5.0 vs. 4.9%) but was not biologically significant. Increasing undegradability of protein significantly increased total protein in milk and casein percentage in milk protein (3.14 vs. 2.86% and 62.11 vs. 58.24%, respectively). Total tract digestibility of nutrients was unaffected by treatment; however, CP digestibility tended to be higher as undegradability increased (67.85 vs. 62.83%).