The in vitro gas coupled with ammonia measurement for evaluation of nitrogen degradability in low quality roughages using incubation medium of different buffering capacity

Abstract

Owing to low N content in low quality roughages, the estimation of in vitro rumen degradable nitrogen (IVRDN) by the method of Raab et al (Br J Nutr 1983 50 569–582) is expected to have a high standard deviation. Incubation of larger amounts of sample will increase the amount of N in the system and decrease analytical errors in the determination of IVRDN. The increase in the amount of sample necessitated an increase in the amount of buffer in the medium. In this study the effect of 30 ml (as is in original method) and 40 ml buffered rumen fluid (containing double the amount of hydrogen carbonate buffer as in the original method) on rumen degradation of N from low quality roughages was evaluated. N degradability of seven cereal straws (barley, millet, oat, rice, sorghum, triticale and wheat) and one grass hay was calculated from the linear regressions of NH3-N concentration vs in vitro gas production. The strength of association (r2) for gas production and NH3-N concentration between 30 and 40 ml system was significantly (P<0·05) different for grass hay and rice straw but not significant for other feeds. Using both systems, the IVRDN for triticale straw was virtually nil, and for others the values obtained using 40 ml system were either similar (oat straw, rice straw, sorghum stover and wheat straw) or higher (barley straw, grass hay and millet stover) than those obtained using 30 ml system. Although no significant difference was found in the standard deviation by increasing the amount of sample, the larger sample has an advantage in that it allows concomitant determination of in vitro apparent and true digestibility. © 1998 SCI.