The effect of form of nitrogen on the efficiency of protein synthesis by rumen bacteria in continuous culture.
The aim of this study was to examine the effect of the form of nitrogen available to mixed rumen bacteria on the efficiency of microbial protein synthesis. A novel, solid substrate, continuous culture fermentor which proved capable of maintaining representative populations of mixed rumen bacteria under steady state conditions, at predetermined growth rates was developed for the study. A series of experiments wherein maize straw, alkaline hydrogen peroxide-treated wheat straw, rye grass or a mixture of maize straw and maize starch were used as substrates were performed in the fermentor. The effect of supplementing these substrates with different forms of nitrogen, namely peptides in the form of a partial hydrolysate of casein, or ammonium salts alone or ammonium salts plus branched-chain volatile fatty acids, on digestion of the substrate and microbial protein synthesis was investigated. Supplementation of maize straw with peptides increased microbial protein synthesis, however this was only significant (P<0.05) when the availability of nitrogen and specific growth rate of the bacteria were not limiting. Organic Matter digestion was not affected by peptide supplementation. Replacement of casein hydrolysate with sunflower oilcake as a source of amino acid nitrogen further increased microbial protein synthesis. The form of nitrogen supplementation did not affect the digestion of constituent cell wall monosaccharides of maize straw. Supplementation of alkaline hydrogen peroxide-treated wheat straw with either branched-chain volatile fatty acids or casein hydrolysate increased the synthesis of microbial protein significantly (P<0.05) and caused a slight, but insignificant increase in the digestion of cellulose-glucose. The efficiency of microbial protein synthesis on rye grass was high, relative to the other substrates, and unaffected by the supplementation of either branched-chain volatile fatty acids or peptides. On media containing high levels of starch, microbial protein synthesis as well as Organic Matter digestion were increased by peptide supplementation, but these differences were only significant (P<0.05) when the bacteria were growing at a high specific growth rate. In all of these experiments, peptide supplementation was accompanied by extensive degradation and deamination of amino acids which offset any increase in microbial protein synthesis. Peptide supplementation therefore resulted in far less efficient overall utilization of protein.