Metabolic Aspects of Urea Utilization by Ruminant Animals

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Abstract
Growing 267-kg steers were fed a low-nitrogen (N), semipurified diet with varying combinations of urea and corn gluten meal to supply either zero, 46 or 92% of the animals' N intake as urea. N balances were 20, 14 and 1 g/day for the respective diets. Fecal and urinary N excretions increased with increasing amounts of urea N in the diet. Other indices of N utilization followed similar trends. Although microbial activity (estimated by redox potential) was higher with urea-containing diets, measurement of ruminal ammonia and partition of the urinary N fraction indicated a loss of N by the animal as a result of ruminal ammonia concentration. Ruminal ammonia peak and biological value were inversely related. Evidently microbial protein synthesis was not comparable in rate to urea hydrolysis. The small change in ruminal ammonia concentration produced from corn gluten meal suggested that this protein is relatively insoluble or has a slow rate of release in the rumen. The inferiority of urea diets can therefore be explained as a combination of urea hydrolysis occurring at a faster rate than synthesis of microbial protein and a portion of less soluble N sources escaping rapid proteolysis in the rumen.