The efficacy of protein supplementation in overcoming urea toxicity in sheep.

1. In the first experiment sheep taken from pasture were given a low-protein diet for six weeks in individual pens. Then, for 1 week, groups were given a supplement of lucerne chaff, safflower meal or lucerne chaff plus safflower meal. In the second experiment eighteen sheep maintained on lucerne chaff rather than pasture were then depleted of protein to a greater extent by feeding on a restricted low-protein diet. Six of the sheep received a supplement of molasses throughout the period of protein depletion while six of the sheep on the basal ration received a supplement of safflower meal after 6 weeks on the low-protein diet. 2. The urea tolerance of the sheep, is indicated by blood ammonia levels after oral dosing with aqueous solutions of urea, was determined after the period of supplementation. "Arginine synthetase" activity (combined activities of argininosuccinate synthetase (EC 6.3.4.5) and argininosuccinate lyase (EC 4.3.2.1) was determined in liver samples obtained by biopsy at various intervals during the experiment. 3. Supplementation for 7 d with 73 g crude protein (nitrogen X 6-25)/d increased the tolerance to urea as indicated by reduced blood NH3 levels, and also increased "arginine synthetase" activity. 4. Giving supplements of molasses delayed the onset of urea toxicity but not the extent of toxicity. 5. It is suggested that short-term feeding of protein concentrates to sheep before giving urea supplements can increase their tolerance to urea.