Discussion of Protein Symposium

Abstract

The 3 invited and 14 poster presenta- tions 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 endo- genous 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 meta- bolic 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. Protein can increase milk yield by providing more amino acids, by increasing available energy, and by altering efficiency of utilization of absorbed nutrients. There is need for additional information on the comparative lactational value of protein sources, amounts of protein that maximize economic return, and the lactational value of protecting proteins and amino acids from ruminal degradation. Adverse effects of excess protein on