Effects of feeding dairy cows protein supplements of varying ruminal degradability

Twenty-five (10 ruminally cannulated) Holstein cows averaging 82 +/- 34 d in milk were assigned to 5 x 5 Latin squares (21-d periods) and fed diets supplemented with one of four different proteins to assess effects on production, ruminal metabolism, omasal flow of N fractions, and degradation rates of protein supplements. Total mixed diets contained (dry matter basis) 44% corn silage, 22% alfalfa silage, 2% urea, and 31% concentrate. Five concentrate mixes were fed: 31% high-moisture shelled corn (HMSC; basal); 9% solvent soybean meal (SSBM), 22% HMSC; 10% expeller soybean meal (ESBM), 21% HMSC; 5.5% blood meal (BM), 25.5% HMSC; and 7% corn gluten meal (CGM), 24% HMSC. Diets averaged, respectively, 15.8, 19.1, 19.7, 20.3, and 19.3% crude protein. Feeding the basal diet reduced intake and yield of milk, fat-corrected milk (FCM), and all milk components compared to the protein-supplemented diets. Milk yield was higher for cows fed ESBM and CGM, fat yield was higher for cows fed SSBM and CGM, but FCM and protein yields were not different among cows fed supplemental protein. Based on omasal sampling, mean in vivo estimates of ruminal degradation rate for the crude protein in SSBM, ESBM, BM, and CGM was, respectively, 0.417, 0.179, 0.098, and 0.051/h (computed using passage rates observed for the small particle phase; mean = 0.14/h), and 0.179, 0.077, 0.042, and 0.026/h (computed using a passage rate of 0.06/h). The in vivo degradation rate computed for SSBM at a passage rate = 0.06/h was similar to that estimated using the inhibitor in vitro method. However, in vivo degradation rates computed at passage rate = 0.06/h for ESBM, BM, and CGM were about two, four, and three times more rapid than those estimated by inhibitor in vitro. Experimental proteins fed in this trial will be used as standards for developing in vitro methods for predicting rates of ruminal protein degradation.