Nitrogen cycling in sheep.

Abstract

Introduction Many of the difficulties in predicting the response of ruminants to supplementation of low-protein diets with non-protein nitrogen (NPN) substances arise because there is a lack of information concerning the quantities of N that recycle between the tissues and the N pools in the digestive tract. Various aspects of N mctabolism in ruminants have been examined but usually the investigations have becn restricted to intensive examination of one component rather than to more comprehensive consideration of the over-all metabolism of N and in particular of urea and ammonia N in intact and feeding animals. Whole animal studies have relied on the use of isotope-tracer techniques and recently major advances have been made using 15N-labelled compounds. In this paper, aspects of N cycling between N pools in the digestive tract and urea in blood in ruminants are discussed in an attempt to define the role of endogenous urea in digestive processes.