Effect of feeding protein supplements of differing degradability on omasal flow of microbial and undegraded protein

Ten ruminally cannulated lactating Holstein cows that were part of a larger trial studying the effects of feeding different proteins on milk production were used in a replicated 5 x 5 Latin square to quantify flows of microbial and rumen-undegradable protein (RUP) in omasal digesta. Cows were fed total mixed rations containing (dry matter basis) 44% corn silage, 22% alfalfa silage, 2% urea, and 31% concentrate. The basal diet contained 31% high-moisture corn; equal N from one of four protein supplements was added to the other diets at the expense of corn: 9% solvent soybean meal (SSBM), 10% expeller soybean meal (ESBM), 5.5% blood meal (BM), and 7% corn gluten meal (CGM). Omasal sampling was used to quantify total AA N (TAAN) and nonammonia N (NAN) flows from the rumen. Estimates of RUP were made from differences between total and microbial N flows, including a correction for RUP in the basal diet. Modifying a spectrophotometric assay improved total purine recovery from isolated bacteria and omasal samples and gave estimates of microbial TAAN and NAN flows that were similar to a standard HPLC method. Linear programming, based on AA patterns of the diet and isolated omasal bacteria and ruminal protozoa, appeared to overestimate microbial TAAN and NAN flows compared to the purine assays. Yields of microbial TAAN and NAN determined using any method was not affected by diet and averaged 32 to 35 g NAN per kilogram of organic matter truly digested in the rumen. On average, National Research Council (NRC) equations underpredicted microbial N flows by 152 g/d (vs. HPLC), 168 g/d (vs. spectrophotometry), and 244 g/d (vs. linear programming). Estimates of RUP (means from the HPLC and spectrophotometric methods) were: SSBM, 27%, ESBM, 45%, BM, 60%, and CGM, 73%. Except for CGM, RUP values averaged about 20 percentage units lower than those reported by the NRC.