The effects of urea supplements on the utilization of straw plus molasses diets by sheep

Abstract

Three experiments with sheep were carried out to investigate the effect of urea, added to a diet of straw plus molasses, on roughage intake and digestion, and on the nitrogen status of the animal. Urea, added to straw and molasses at the level of 3% of the amount of straw, increased the ad libitum food intake, rate of cellulose (cotton thread) digestion in the rumen, and rate of passage of food through the gut. When different amounts of urea were fed, the highest levels of intake, rate of cellulose digestion, and rate of passage occurred with 8–16 g urea per sheep per day. Increasing the amount of urea fed to 32 g per day caused significant decreases in rate of passage and intake, within diets containing urea. These were not accompanied by significant changes in rate of cellulose digestion in the rumen. With diets of straw and molasses, with and without urea, crude fibre digestibility was positively correlated with the rate of cellulose digestion in the rumen. Voluntary intake of these diets was positively correlated with rate of passage. When the effect of rate of passage was eliminated, voluntary intake was not significantly correlated with the rate of cellulose digestion in the rumen. Under the feeding conditions used in these experiments, once sufficient urea had been added to the diet to bring the animal into a small, positive nitrogen balance, additional dietary nitrogen supplied as urea was practically all excreted in the urine. It is concluded that, under these feeding conditions, the primary function of a urea supplement is to enable an animal to maintain nitrogen equilibrium rather than store significant amounts of nitrogen in the body.