Degradation of Supplemental Proteins in the Rumen 1,2

Three Holstein steer calves (179 kg) were fitted with reentrant cannulas at the proximal duodenum. The animals were used in a series of metabolism experiments designed to measure duodenal passage of nonammonia and nonmicrobial N, and to evaluate the extent of degradation of supplemental protein within the rumen. When casein-, soybean meal-, cottonseed mealand corn gluten meal-supplemented diets were fed at 3,000 g/day, estimates of rumen protein degradation were 103, 85, 76 and 54%, respectively. When soybean meal, cottonseed meal, linseed meal, corn gluten meal and meat and bone meal were fed at 4,000 g/day, rumen protein degradation estimates were 82, 39, 56, 39 and 30%, respectively. A prediction equation was developed for estimating rumen bypass on the basis of solubility as well as dacron bag rumen digestion rate estimates. The correlation between predicted values and those measured in vivo was high (r = .984, P<.01). Microbial efficiency (grams microbial protein/100 g organic matter fermented) averaged 15.1, with a range of 11 to 19. Microbial efficiency was closely related to intake of digestible organic matter and rate of passage of nonmicrobial organic matter from the rumen. (