Metabolism of urea nitrogen and adaptation response trends in the growing wether and steer.
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Four wethers of average weight 33 kg were fed for 60 days on hay, straw, beet pulp, maize starch, glucose, molasses, soya bean oil, urea, salt mixture and vitamins. The diet was then readjusted, with crystalline urea to provide 50% of total N, mixed in the feed or sprayed on it, or given by drench twice a day. Retention of urea N was 55, 64 and 60% in wethers getting urea mixed in the feed, sprayed on it or by drench, respectively. Average daily liveweight gain with spraying was 104 g, with mixing and by drench it was 59 and 57 g. Mixing caused significantly greater excretion of urea N than spraying. Excretion of urea was negatively correlated with excretion of NH3. Wethers on the basal diet excreted less urea N in urine and it was less than NH3N. Two groups of young bullocks of average weight 193 kg were given a diet similar to that described but excluding hay and glucose. Urea to provide 71% of total N was mixed with the diet, sprayed on it or given by drench. Spraying gave average daily gain of 223 g and mixing 148 g. There was no difference in N retention. A significant decline in excretion of urea N gave a decrease of 82% in excretion of N in urine. By changing daily intake of N from 360 to 133 mg and back to 360 mg in sheep sudden changes in excretion of urea N were induced; this was regarded as a short-term adaptation. There were slow changes in excretion of non-urea N and these were regarded as indicative of a long-term adaptation in N retention as long as the decrease in NH3 and residual N fractions was greater than the corresponding increase in urea N.-B. A. O.