LACTATIONAL RESPONSES TO RUMINALLY UNDEGRADABLE PROTEIN BY DAIRY-COWSFED DIETS BASED ON ALFALFA SILAGE

Lactational responses to protein supplementation of diets containing 6 0% of DM as alfalfa silage were evaluated. Sixty multiparous 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 isonitr ogenous (19.4% CP) diets. Diets were formulated with soybean meal, a b lend 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 o f 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 pro tein percentage (2.83%), milk urea (7.66 mM), or plasma urea (8.91 mM) . However, cows fed diets supplemented with soybean meal had higher DM I (26.2 vs. 24.7 kg/d), milk yields (40.4 vs. 39.1 kg/d), and milk pro tein (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 d iets containing a blend of animal by-products. The lack of response to ruminally undegradable protein was partially caused by higher than pr edicted DMI (5 to 15% above NRC predictions); all diets provided at le ast 1.3 kg of ruminally undegradable protein, and there was no benefic ial effect from ruminally undegradable protein intake increases to 1.6 kg/d.