UTILIZATION OF ENDOGENOUS AND DIETARY UREA IN THE LARGE-INTESTINE OF THE MATURE HORSE

The dynamics of N metabolism in mature horses were investigated when t hey were fed on a low-N diet or the same diet supplemented with suffic ient urea or soyabean meal to meet their theoretical N requirements, T here were no differences in DM, organic matter or neutral-detergent-fi bre digestibilities for the three diets, N digestibilities and digesti ble-N intakes were similar for the urea- and soyabean-supplemented die ts and very low for the low-N diet, For all three diets plasma urea wa s degraded in the digestive tract to NH3 which was utilized by the bac terial population as a N source. NH3 was absorbed from the large intes tine into the blood and converted to urea, NH3 was also incorporated i nto plasma proteins, The horses fed on the low-N diet degraded a great er proportion of endogenous urea in the digestive tract than did horse s fed on the urea- or soyabean-supplemented diets. However, the horses fed on the urea diet had the highest degradation rate of urea. The qu antity of urea degraded in the digestive tract of horses fed on the ur ea-supplemented and the low-N diets could not compensate for a lack of dietary crude protein in these diets, The horses were in a negative N balance when fed on the low-N and urea-supplemented diets and a posit ive N balance when fed on the soyabean-supplemented diet. Dietary urea supplementation did not benefit the horses.