Effect of Ammonia Concentration on the Composition, Hydrolytic Activity and Nitrogen Metabolism of the Microbial Flora of the Rumen

The mean NH 3 concentration in the rumen of sheep fed whole barley (0d̀8 kg/d) by continuous feeders was increased from 6d̀1 to 13d̀4 HIM by supplementing the feed with urea (30 g/kg). This change caused a 90% increase in the rate of degradation of rolled barley, and smaller increases in the rates of degradation of protein and plant fibre in the rumen. The total viable count and numbers of pectinolytic bacteria in rumen fluid increased with the urea supplement. Enzyme studies indicated that NAD‐linked glutamate dehydrogenase was the main pathway of NH 3 assimilation by rumen bacteria at both NH 3 concentrations. Glutamate was the main free amino acid found in the rumen at low NH 3 but, despite the low activity of alanine dehydrogenase and glutamate‐pyruvate aminotransferase, alanine was the principal amino acid at high NH 3 concentrations. Hydrolytic rumen bacteria may require the higher NH 3 concentration either for effective NH 3 assimilation by an unknown mechanism involving alanine or for full expression of enzyme activity.