Nitrogen metabolism in the ovine stomach: 1. The transfer of urea from the blood to the rumen

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Abstract
SUMMARY: The effect of single, intravenous injections of urea on the concentration of ammonia in the rumen has been examined in conscious and anaesthetized sheep.In conscious sheep maintained on a diet providing 11·4 g nitrogen/day, the net increase in concentration of rumen ammonia after the injection of urea showed negative correlations with the pre-injection concentrations of rumen ammonia and of plasma urea. The maximum concentration of ammonia attained in the rumen after the injection wasx 16·6 ± 0·28 mg NH3-N/100 ml. No such correlations were found in conscious sheep given a diet providing 20·7 g N/day. In addition, the maximum concentration of ammonia attained in the rumen after the injection of urea was significantly lower at 10·7 ± 0·91 mg NH3-N/100 ml, and the net increases in concentration were smaller.Starvation for one day reduced the net increment in rumen ammonia concentration after urea injection in animals given the lower-N diet, to the same low level observed in fed or starved sheep maintained on the higher-N diet.Differences were observed in the transfer of blood urea to the rumen in experiments with anaesthetized sheep which had been maintained on the lower-N diet for long or short periods beforehand. Animals which had been left to graze, and then housed indoors and given the lower-N diet for a short period before the experiment, showed much smaller increases in rumen urea plus ammonia concentration after the injection of urea than sheep which had been given the same diet for a long period. There was no difference in the response of the salivary secretions to the urea injections.