Effect of rumen ammonia concentration on in situ rate and extent of digestion of feedstuffs.

Three dry Holstein cows fitted with rumen fistula were fed a 7.4% crude protein diet consisting of 47.4% corn, 50% cottonseed hulls, and 2.6% minerals and vitamins during a 44-d experiment. Treatments consisted of rumen infusion, 0, 33, 67, and 100 g/d of urea nitrogen applied in a four period Youden Square design. Increasing urea infusion increased rumen fluid ammonia nitrogen from 4.3 to 25.0 mg/dl. Estimated effective dry matter degradation based on in situ rates of digestion were increased from 67.9 to 74.4% for corn and 77.5 to 80.3% for soybean meal with maximums at 100 g/d infused urea nitrogen. Up to 67 g/d infused nitrogen increased dry matter degradation of corn gluten feed from 67.0 to 71.4% and cottonseed meal dry matter degradation from 56.7 to 60.1%. Alfalfa hay dry matter and neutral detergent fiber degradation were not increased by urea infusion. Minimum rumen ammonia required in feeds in this trial were pooled with literature data and suggest that lowest ammonia concentrations required for maximal digestion was a function of the rumen fermentability of the diet or feed. The equation: minimum ammonia concentration (mg/dl) = .452 fermentability % - 15.71, accounted for 50% of the variation in minimum ammonia requirements. We conclude that rumen ammonia concentrations required for maximum digestion are not constant but rather are a function of fermentability of the diet.