Comparison of protein fermentation characteristics in rumen fluid determined with the gas production technique and the nylon bag technique

In this study, a modified version of the gas production technique was used to determine protein fermentation characteristics in rumen fluid of 19 feedstuffs. Performing the incubations in a N-free environment, and with an excess of rapidly fermentable carbohydrates, made N the limiting factor to microbial growth, and so gas production profiles reflected the availability of N from the feed samples. Results showed that fermentation of protein in rumen fluid can be determined with this modified gas production technique, and that there were distinct differences in protein fermentation between the feed samples. Availability of protein for fermentation was highest in wheat, potato pieces and lupin, and lowest in Rumiraap, a formaldehyde treated rapeseed meal, palm kernel expeller and brewery grains. The protein degradation characteristics of the 19 feed ingredients were also determined with the in situ nylon bag technique. With the obtained results, the amount of rumen escape protein (REP) was calculated for each feedstuff. The results showed that the rate of degradation ranged from 0.010/h for Rumiraap to 0.151/h for wheat. The amount of REP ranged from 197 g/kg CP for lupin to 840 g/kg CP for Rumiraap. Comparing the gas production results with the results obtained with the nylon bag technique showed that there was a good relationship between the gas production after 12–25 h of incubation and the calculated amount of REP (r2 = 0.83–0.85). The results show that the adapted gas production technique, being depleted of N and using an excess of rapidly fermentable carbohydrates, is suitable to recognize differences in N availability between feed samples and can be used as an alternative to the nylon bag technique and other in vitro techniques.