Abstract:
The urinary excretion of urea, the fate of an intravenous dose of urea, overall nitrogen metabolism and the composition of rurnen and abomasal samples were studied in crossbred sheep given a basal roughage diet to which supplements of starch, casein or formaldehyde-treated casein were added. Urea excretion was closely related to plasma urea level (r = 0.935) and to the filtered load of urea (r = 0.957). The amount of urea reabsorbed by the kidney increased as'the amount filtered increased; the relationship was improved when values obtained at urine flow rates less than 1 rnl/rnin were omitted. When sheep were given the starch and treated casein diets, the rate at which they cleared a small dose of urea from their plasma was greater than the renal clearance rate; only 75% of a large dose was recovered in the urine. The capacity to dispose of exogenous urea by extra-renal pathways was apparent only when rumen ammonia nitrogen levels were less than about 200 mg/l. Formaldehyde treatment reduced nitrogen digestibility but reduced urine nitrogen excretion to a greater extent so that nitrogen retention increased. Treatment was associated with reduced plasma urea levels, reduced rumen and abomasal ammonia levels, reduced total VFA levels, increased proportions of acetic acid and decreased proportions of isobutyric, isovaleric and n-valeric acids, and more protein in abomasal contents.