INFLUENCE OF LEVEL OF UREA AND METHOD OF SUPPLEMENTATION ON CHARACTERISTICS OF DIGESTION OF HIGH-FIBER DIETS BY SHEEP

Effects of level and method of urea supplementation on N utilization and characteristics of digestion of a N-deficient (.85% N), citrus pulp-based diet were evaluated in a 4 x 4 Latin square design experiment, using four ruminally and intestinally cannulated crossbred (D'man x Sardi) rams. Two levels of urea (15.1 and 8.8 g/d, providing total N intakes of 12.0 and 9.1 g/d, respectively) and two methods of supplementation (urea mixed with the diet or continuously infused in the rumen) were compared in a 2 x 2 factorial arrangement of treatments. Ruminal pH varied between 6.2 and 7.0 across treatments and tended (P > .10) to be lower in animals receiving urea infusion. Ruminal ammonia concentrations were lower (P < .01) and more stable with urea infusion than with urea feeding. Ruminal VFA concentrations and profiles were unaffected (P > .10) by treatments. Site and extent of OM and fiber digestion were not influenced (P > .10) by either level of urea in the diet or method of supplementation. Average ruminal OM and ADF digestion were 53.1 and 64.5%, respectively. Total tract digestion values averaged 75.9 and 73.5%, respectively. With the exception of N efficiency (duodenal nonammonia N/total N intake) that was 31% lower (P < .10) for the high- than for the low-urea treatment, there were no treatment responses or interactions (P > .10) on characteristics of N utilization. Microbial N flow to the small intestine averaged (across treatments) 9.0 g/d and constituted 87% of the nonammonia N leaving the abomasum. It was concluded that neither OM digestion nor microbial net synthesis or efficiency is enhanced by high levels of ruminal ammonia or by continuous N release in the rumen.