Regulation of Urea Recycling into the Gastrointestinal Tract and Ammonia Metabolism in Ruminants

Abstract

The primary objective of this study was to determine the extent ruminants control urea recycling to the gastrointestinal tract (GIT), independent of rumen microbial metabolism. Urea kinetics were determined by continuous infusion of [15N15N]urea (4 levels) to jugular vein of sheep (n = 4; 28.1kg) fed a low protein (6.8%) diet. Although urea recycling to the GIT (P < 0.001) increased with each level of urea, the proportion (P < 0.001) and amount (P < 0.001) of recycled urea returning as ammonia to liver for ureagenesis also increased. In consequence a decreasing proportion (P = 0.003) of recycled urea was used for microbial protein synthesis. The present study suggests that ruminants have a large capacity to partition urea-N to the GIT. Thus, rather than up-regulating urea recycling, there appears to be more potential to improve N efficiency in ruminants by manipulating the rumen environment to optimize capture of recycled N.