Discussion of Protein Symposium

The 3 invited and 14 poster presentations provided information on digestion and metabolism of protein and effects of protein on milk yield. Flow of amino acids to the small intestine is determined by ruminal degradation of dietary protein and synthesis of microbial protein. These are dynamic processes that cannot be described by static transfer coefficients. Flux of ruminal ammonia is the primary determinant of nitrogen economy. Sources of ammonia include degraded dietary protein, degraded microbial protein, saliva, protozoal excretion, and endogenous protein. Ammonia leaves the rumen by absorption, by incorporation into microbial cells that leave the rumen, and in fluids passing out of the rumen. Intestinal absorption of amino acids appears relatively constant over a wide range of diets. Proportions of fecal protein that are of dietary and of metabolic origin are not defined well. Metabolic fecal protein is the most variable factor in the new protein systems. Information on utilization of absorbed amino acids is limited.