Utilization of blood urea in ruminants

Utilization of injected urea and urea transfer from blood to rumen were investigated. First, urea ( ca. 5 mmole urea-N/kg body weight) was injected intravenously into mature sheep receiving a low-protein, carbohydrate-supplemented ration. Part of the injected urea was recovered in urine as excess over basal excretion. 52% (S.D. = 10) of injected urea was not recovered in urine nor remained in body fluids and presumably was utilized in rumen protein synthesis. Without dietary carbohydrate supplements utilization decreased to 22% (S.D. = 5). Second, ingesta of the rumeno-reticular cavity were replaced with saline; and accumulation of ammonia, representing a hydrolytic product of urea, was measured. Concurrent absorption of ammonia was estimated using arteriovenous urea and ammonia concentration data. Total urea-N transfer, the sum of ammonia-N accumulation and absorption plus salivary urea-N, for sheep was 5.2 mmole urea-N/hr. (S.D. = 0.8), of which 0.3 mmole/hr. was in saliva. Similar results were obtained in the goat.