THE DEGRADATION AND UTILIZATION OF ENDOGENOUS UREA IN THE GASTROINTESTINAL TRACT OF RUMINANTS: A REVIEW

Abstract

Urea transferred from the blood to the digestive tract of ruminants is potentially an important source of nitrogen for microbial growth in the reticulorumen and hind gut. Early attempts to assess the quantitative importance of urea transfer did not distinguish between its reticuloruminal and hind gut components. However, recent advances in tracer methodology have facilitated this partition, and a number of estimates have been made, principally using sheep on roughage diets. For some roughage diets, the rate of urea transfer to the rumen of sheep was small and was attributable to salivary urea, whereas for other diets substantial amounts of urea appeared to be entering the rumen across the rumen epithelium in addition to that carried in saliva. Regression analysis indicated that the rate of transfer of endogenous urea to the rumen of sheep given those diets was associated with the concentration of rumen ammonia and of plasma urea, and with the amount of organic matter digested in the rumen. Experimental evidence supporting the validity of this concept is discussed. Relationships are presented which show that the clearance of plasma urea to the rumen is inversely proportional to the concentration of rumen ammonia in both sheep and cattle, but that clearance is increased by the addition of grain or sucrose to the diet. It is concluded that transfer of urea across the rumen epithelium may contribute significantly to the nitrogen economy of the microorganisms, but the mechanism by which such transfer is controlled by the ruminant remains obscure.