Relationship between ruminal ammonia and nonprotein nitrogen utilization by ruminants. III. Influence of intraruminal urea infusion on ruminal ammonia concentration.

In three trials, we studied the effect of incremental amounts of intraruminally infused urea on mean ruminal ammonia concentration of steer fed at 2-h intervals. Basal rations contained these percentages of crude protein and total digestible nutrients (dry matter basis); Trial I, 11.1 and 81; Trial II, 6.0 and 54; Trial III, 6.5 and 58. Mean ruminal ammonia concentration reached 5 mg ammonia nitrogen/100 ml rumen fluid at crude protein equivalents of 12.0, 9.3, and 9.4% in I, II, and III. Once ruminal ammonia began to accumulate, there was a linear relationship between intake of urea and mean concentration of amino acids of plasma, serving as an indirect measure of amino acid absorption from the intestine, was not increased by increased intake of urea in III. Results of this experiment support the concept from in vitro data that microbial protein synthesis is unaffected by ruminal ammonia concentration in excess of 5 mg ammonia nitrogen/100 ml rumen fluid.