Effects of source and amount of protein on ruminal fermentation and passage of nutrients to the small intestine of lactating cows

Four Holstein cows fitted with ruminal and duodenal cannulas were used in a 4 x 4 Latin square to investigate the effects of source (corn gluten meal or soybean meal) and amount (14.5 or 11.0%) of CP on ruminal fermentation, passage of nutrients to the small intestine, and animal performance. Cows wee fed for ad libitum intake a diet of 60% corn silage and 40% concentrate on a DM basis. The treatments, arranged in a 2 x 2 (source x amount of CP) factorial, were 1) 14.5% CP, soybean meal; 2) 11.0% CP, soybean meal; 3) 14.5% CP, corn gluten meal; and 4) 11.0% CP, corn gluten meal. Digestion in the rumen of OM, starch, ADF, and NDF was not affected by source or amount of CP in the diet. Total VFA and NH3 concentrations in ruminal fluid were increased by feeding diets that contained 14.5% CP or soybean meal. FLows of non-NH3 N and amino acids to the duodenum were greater in cows fed the 14.5% CP diets because of a greater flow of non-NH3 nonmicrobial N to the duodenum. Larger amounts of lysine passed to the duodenum when cows were fed soybean meal compared with corn gluten meal. Microbial N flow to the duodenum and efficiency of microbial growth were not affected by treatments, suggesting that ruminal NH3 concentration was not limiting for maximal microbial protein synthesis. Feeding 14.5% CP diets increased the production of milk (29.5 vs. 26.8 kg/d) and milk protein compared with 11.0% CP diets, possibly because of greater passage of amino acids to the small intestine. Feeding soybean meal to cows increased production of milk protein compared with feeding corn gluten meal, possibly because more lysine passed to the small intestine.