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Evaluation of Urea, Biuret, Urea Phosphate and Uric Acid as NPN Sources for Cattle

United States Department of Agriculture, ARS, Animal Husbandry Research Division, Beltsville, Maryland

Urea, biuret, urea phosphate and uric acid were evaluated as sources of dietary nitrogen in purified diets for steers. In vivo studies with steers fed urea indicated that the ruminal microorganisms were essentially unable to degrade biuret to ammonia but degradation of uric acid to ammonia occurred to a greater extent. When the steers were fed these nonprotein nitrogen (NPN) sources for 21 days ruminal degradation to ammonia was more pronounced, especially with uric acid. Metabolism results indicated that the apparent digestibility of dry matter and gross energy were significantly greater when uric acid was compared with biuret. Acid detergent fiber digestibility was significantly less when steers were fed biuret as compared with the other NPN sources. Nitrogen retention (% of intake) was 18.4, 16.9, 12.3 and 23.1 for urea, biuret, urea phosphate and uric acid, respectively. Urinary excretion of biuret and uric acid was significantly greatest when the steers were fed these NPN sources, but while the urinary levels of uric acid were low in comparison with intake, approximately 25% of the ingested biuret was excreted in the urine. Twenty amino acids and other ninhydrin-positive compounds were significantly affected by NPN source, time after feeding or an interaction between these. Apparent blood plasma biuret was noted in significantly greater quantities, whereas blood plasma urea was present in significantly lesser quantities when steers were fed biuret. Blood plasma concentration of urea and several of the essential amino acids were significantly lower 4 hours after feeding compared with before feeding.