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 they were fed on a low-N diet or the same diet supplemented with sufficient urea or soybean meal to meet their theoretical N requirements. There were no differences in DM, organic matter or neutral-detergent-fibre digestibilities for the three diets. N digestibilities and digestible-N intakes were similar for the urea- and soyabean-supplemented diets and very low for the low-N diet. For all three diets plasma urea was degraded in the digestive tract to NH3 which was utilized by the bacterial population as a N source. NH3 was absorbed from the large intestine into the blood and converted to urea. NH3 was also incorporated into plasma proteins. The horses fed on the low-N diet degraded a greater proportion of endogenous urea in the digestive tract than did horses fed on the urea-or soyabean-supplemented diets. However, the horses fed on the urea diet had the highest degradation rate of urea. The quantity of urea degraded in the digestive tract of horses fed on the urea-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 positive N balance when fed on the soyabean-supplemented diet. Dietary urea supplementation did not benefit the horses.