Validation of in situ nitrogen degradation measurements: comparative proteolytic activity of solid-adherent microorganisms isolated from rumen content and nylon bags containing various feeds

Four different feeds (corn, rapeseed meal, untreated and formaldehyde treated soybean meal) were incubated in nylon bags in the rumen of three cannulated dairy cows fed twice daily with a 70:30 forage:concentrate diet. After 2, 6 and 23 h incubation time, the solid-adherent microbial population was isolated from both nylon bags and rumen content, and its proteolytic activity against 14C casein was measured. The differences in proteolytic activity between rumen content and bags were slight. In the bags, the activity varied with the incubated feeds, but differences between feeds were observed only for longer incubation times, and thus would not limit the use of the in situ method to characterize feeds nitrogen degradability.