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Discussion of Protein Symposium

School of Veterinary Medicine, University of Pennsylvania¹, Kennett Square 19348

ABSTRACT

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.

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 production and reproduction warrant further investigation.

New systems of protein evaluation provide a framework for formulating diets from digestive and metabolic processes. Metabolically, dairy cows are in dynamic state with patterns of nutrient use and demand changing with the lactation cycle. Because information on digestion and metabolism of protein in dairy cattle is limited, we rely heavily upon data from nonlactating cattle and sheep. As additional information becomes available, dynamic models of protein nutrition will be used to provide recommendations for specific phases of the lactation cycle rather than for milk production per se.

¹ Nutrition Section, Department of Clinical Studies at New Bolton Center.