Effects of ammonia concentration and microbial population on in vitro degradation of 14C-labelled dietary proteins

Abstract

Rumen fluid from two nonlactating cows fed barley silage and rolled barley grain based concentrates (75:25 on a dry matter basis) was incubated for 0.5, 1, 2, 3, and 4 h with 14C-labelled soybean meal (SBM), fish meal (FM) and corn gluten meal (CGM) to examine the effects of ammonia concentration and protein solubility on rate and extent of protein degradation by total mixed ruminal microorganisms (TMM) or mixed ruminal bacteria (MB). Proteins were labelled by reductive methylation. Ammonia concentration in control ruminal fluid was 4.0 mg dL−1; graded levels of 1 M (NH4)2SO4 were added to achieve ammonia concentrations of 10, 20, and 30 mg dL−1 ruminal fluid. Soluble protein was extracted with borate-phosphate buffer (pH 7.8). Removal of protozoa reduced the extent of protein degradation. However, relative to bacteria, protozoa appeared to play a minor role in protein degradation. Rate and extent of protein degradation was not influenced by ammonia concentration, but was highly correlated with protein solubility. Therefore, it is concluded that ammonia concentration is not the primary factor regulating proteolytic activity of rumen microorganisms, rather their activities may depend upon the solubility and physico-chemical properties of proteins. Key words: In vitro, ammonia concentration, protein degradation, microbial population