Submitted to: Journal of Dairy Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/14/2002
Publication Date: 10/14/2002
Citation: Interpretive Summary: Extensive breakdown of feed protein in the rumen, the first compartment of the cow's stomach, can result in inefficient use of dietary protein and depress milk production. This is because the rumen microbes, which both synthesize and degrade protein, often degrade more feed protein then they synthesize. Feed proteins that are slowly degraded in the rumen (largely escape the rumen) must still be digestible in the small intestine to be used by the cow. Twenty-five dairy cows eating a typical diet were fed no protein supplement other than urea (basal ration) or equal amounts of protein from one of four common supplements: soybean meal (the major protein fed to dairy cows in the U.S.), expeller soybean meal (a heat-treated meal), corn gluten meal, or blood meal (a protein known to largely escape the rumen undegraded). Feeding the basal diet resulted in lower intake and depressed yield of milk and milk components compared to feeding any one of the protein supplemented diets. Among cows fed one of the four supplements, protein yield was not different. However, milk yield was higher when cows were fed expeller soybean meal and corn gluten meal. Rates of protein degradation in the rumen were determined for the four supplements: rates for blood meal and corn gluten meal were slowest, that for expeller meal was intermediate, and that for solvent meal was fastest. That milk production was not greater on blood meal than on solvent soybean meal, even though blood meal was degraded only half as fast, indicated that digestion of blood meal protein may be impaired in the small intestine. This research indicates that utilization of supplemental protein is determined only partly by rate of degradation and extent of escape from the rumen. This means that researchers must also develop methods to measure intestinal digestibility, to complement determination of rumen degradation, to properly evaluate feed proteins for dairy cows.
Technical Abstract: Twenty-five (10 ruminally cannulated) Holstein cows averaging 82 ± 34 days in milk were assigned to 5x5 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. Dietary concentrates fed were: basal (31% high moisture shelled corn (HMSC)]; SSBM (9% solvent soybean meal; 22% HMSC); ESBM (10% expeller soybean meal, 21% HMSC), BM (5.5% blood meal; 25.5% HMSC); and CGM (7% corn gluten meal, 24% HMSC). Diets contained, respectively, 15.7, 19.1, 19.7, 20.3 and 19.3% CP. Feeding the basal diet resulted in lower 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 SSBM, ESBM, BM, and CGM were, 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 2-, 4-, and 3-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.