The effect of urea on the utilization of low quality roughage by the ruminant.

Abstract

The Effect of Urea on the Utilization of Low-Quality Roughage by the Ruminant J. B. COOMBE* and D. E. TRIBE* In Australia, the main interest in urea. feeding to ruminants is the possibility of using this substance as a protein supplement for roughages so high in fibre and low in protein that they constitute sub-maintenance rations for animals grazing them. Several workers have shown that the loss of weight of ruminants fed such roughages can be prevented, or reduced, by adding a supplement of urea and a source of available energy. (Clark & Quin, 1951; Williams, Pearce, Delaney & Tribe, 1959; Coombe, 1960; Morris, 1958.) In all of these experiments, one of the consequences of urea feeding was the increased intake of roughages. Clark & Quin (1951) postulated that this was due to an increase in the rate of cellulose digestion, rather than an increase in actual digestibility. The present paper describes the effects of urea on the digestibility and intake of low quality roughage, the rate of cellulose digestion in the rumen, the rate of passage of food through the alimentary tract, and the status of nitrogen and Vitamin B complex in the rumen. Digestibility trials were run with 4 sheep, fed straw (crude protein 3% ) plus molasses, or straw, molasses and urea, and the results of these are summarized in Table 1. TABLE 1 Effect of Urea Supplementation on the Digestibility of Oaten Straw and Molasses. From the results in Table 1 it can be seen that the'digestibility of theI dry matter of the oaten straw ration was not significantly affected by urea supplementation, although, in three out of four sheep, the average daily dry matter intake was significantly increased. It is interesting to note that urea appears to have increased digestibility in the only case where dry matter intake was not increa' sed. In the rate of passage studies, the technique of Castle (1956) was used, with modifications in the counting technique as suggested by Arnold (Personal Communication). The marker used was c: School of Agriculture, University of Melbourne. 83 about 30 g. of the chopped oaten straw, stained with brilliant green. Supplementation with urea generally caused a shortening of the mean retention time of an average of about IO hours, viz., from 59 hours to 49 hours. Results of four sheep are shown in Table II. The disappearance of pure cellulose thread in the rumens of two fistulated sheep, was used to measure the rates of cellulose digestion in the rumen (Balch & Johnson, 1950). The results are expressed as the time taken for 50% loss of weight of the thread in the rumen, and are summarized in Table II. TABLE II Effect of Urea, Added to Oaten Straw and Molasses, on the Rate of Passage of Food and Rate of Cellulose Digestion. These results indicate that an increase in ad libitum food intake produced by the addition of urea to a low quality roughage is probably due to an increased rate of cellulose digestion. However, individual sheep have shown marked exceptions to this generalization; in some instances, ad libitum intake has not been increased with urea e.g. sheep 373 in Table 1; nor has urea in every case increased rate of passage or rate of cotton thread digestion in the rumen. At present we cannot explain how or why these exceptions occur. The nitrogen status of the rumen was studied by following the changes in NHS-N concentration in rumen liquor after feeding rations of straw, straw plus molasses, straw plus urea, and stra,w plus molasses and urea. Urea caused marked increases in rumen NH3-N concentration, while the addition of molasses, with or without urea, caused a decrease in rumen NHs-N concentration. Rumen pH levels were also lowered by the feeding of molasses. Finally, the effect of urea feeding on the synthesis of thiamine, riboflavin and nicotine acid in the rumen has been studied in sheep on roughage diets differing in energy, nitrogen and vitamin contents. On diets rich in vitamins, there was little rumen synthesis. On deficient diets, however, active synthesis occurred, and its level was affected by several factors, one of the most important being the supply of nitrogen in the diet. On high-energy rations, urea-N was an effective as casein-N in promoting synthesis of thiamine and riboflavin. Even on low-energy rations, a three-fold increase in the synthesis of thiamine, riboflavin and nicotinic acid was produced by the addition of urea. 84 REFERENCES Balch, C. CT., Johnson, V. W. (1950).-Brif. J. Nutrit. 4. 389. Castle, Elizabeth J. (1956).-Brit. J. Nutrit, 10. 15. Clark, R.; Quin. J. I. (1951) .-Onderstepoort J. Vet. Res. 25. 93. Coombe, J. B. (1959).- J. Aust. Inst. Agric. Sci. 25: 299. Morris, J. G. (1958).-Qd. J. Agric. Sci. 15 161. Williams, N. M.; Pearce, G.; Delaney, M.; Tribe, D. E. (1958) .Emp. J, Exp. Agric. 27, 107. DISCUSSION Dr. J, P. Hogan, (N.S.W.) asked how the levels of rumen volume were estimated ? Answer .-They were measured in three ways: 1. Direct measurement when two of the experimental animals were slaughtered. 2. By the polyethylene glycol technique. 3. A number of animals due for slaughter were fed similar diets to those described and their rumen volumes were measured directly. ' R. J. Moir (W.A-) asked for information on the spraying of dry roughage with urea and molasses. Answer.-This treatment maintained body-weight of sheep, while the control sheep lost 21b. per day. The pasture consisted mainly This experiment of Wimmera rye-grass and subterranean clover. was short because of a fall of rain. Dr. Tribe pointed out that the inclusion of molasses in the ration reduced the levels of free a.mmonia in the rumen and kept the pH level low. 85