RUMEN DIGESTION AND URINARY-EXCRETION OF PURINE DERIVATIVES IN RESPONSE TO UREA SUPPLEMENTATION OF SODIUM-TREATED STRAW FED TO SHEEP

The present study examined the effect of urea-N supplementation of a N -deficient diet on digestion and metabolism in the rumen. Five Rasa Ar agonesa ewes, each fitted with a rumen cannula, were offered alkali-tr eated barley straw ad lib. alone or supplemented continuously via the cannula with four levels of urea-N (3, 6, 9 and 12 g/d). Rumen NH3 con centrations increased in response to urea infusion (6-128 mg/l; P < 0. 001). At the highest level of rumen NH3 concentration there was a sign ificant increase, compared with the unsupplemented treatment, in dry m atter (DM) intake (846-1206 g/d; P < 0.001) and apparent digestibility of DM (0.38-0.43), organic matter (0.38-0.45) and neutral-detergent f ibre (0.41-0.49; P < 0.01). Rumen outflow rates of particulate matter and potential DM disappearances, assessed using nylon bags, were not a ffected by the experimental treatments, although fractional rate. of D M disappearance increased significantly with increasing levels of urea infusion (2.4-4.6 per h). Urinary excretion of total purine derivativ es increased with N supplementation, although the response was exclusi vely due to an increase in allantoin excretion (26.9-66.4 mg/kg live w eight (W)0.75 per d; P < 0.001). Xanthine, hypoxanthine and uric acid excretion rates were constant, averaging 1.8 (SE 0.17); 5.4 (SE 0.21) and 7.2 (SE 0.36) mg/kg W0.75 per d respectively. The maintenance of a minimum rumen NH3 concentration (approximately 50 mg/l) was necessary to avoid significant reductions in DM intake and fermentation rate. H igher levels, however, may further increase microbial N flow at the du odenum, as suggested by the response in urinary allantoin excretion ov er the range of rumen NH3 concentrations.