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Quantitative Aspects of Nitrogen Utilization in Ruminants

The Ian Clunies Ross Animal Research Laboratory, Division of Animal Physiology, C.S.I.R.O., P. O. Box 239, Blacktown, N.S.W., Australia 2148

ABSTRACT

Conclusion: The information in this paper has been presented to indicate the types of calculations that are now becoming possible with the advent of new information on quantitative aspects of protein metabolism. No particular claims are being made for the accuracy of the present calculations because the data on which they are based have in several instances been extra-polated from one species of ruminant to another; indeed most of the production data, based as they are on average figures for the composition of milk and meat, could be seriously astray. However these calculations are by no means without value. For instance, it is possible to calculate that the semipurified diets used by Virtanen (56) supported the synthesis of about twice the amounts of essential amino acids that appeared in milk. Conversely, molasses-fed beef cattle in Cuba (43) could not have synthesized their requirements of essential amino acids from urea on the basis of the amount of fermentable carbohydrate ingested, and as was found in practice, a protein supplement had to be provided.

The use of these rather primitive calculations is justified if for no other reason than to point to the gaps in our knowledge. The calculations indicate for instance the need to determine the composition of tissue losses in the lactating cow, and of tissue gains in the cow during late pregnancy. They indicate the need to study the efficiency with which , inu acids absorbed from the small intestine ttre incorporated into products. In view of the fact that digestible crude protein is not necessarily a good indicator of the amounts of amino acids derived from a diet, the whole question of energy-protein relationships in the ruminant urgently needs reappraisal. New techniques measure the supply of the major metabolites: volatile fatty acids, amino acids, and long chain fatty acids; from the digestive tract. However, the need also exists to determine preferential utilization of different substrates for maintenance and various forms of production. The benefits arising from such studies will lead not only to greater efficiency of conversion of feed into ruminant protein but also to the development of more realistic systems for the nutritional evaluation of diets.