Effects of urea infused into the rumen on liquid- and particle-associated fibrolytic enzyme activities in steers fed low quality grass hay

Effects of urea infusion on the endoglucanase (CMCase) and xylanase activities in solid and liquid fractions of rumen contents in steers were investigated. Three steers were fed timothy hay once daily for 46 days that were divided into two periods, control period followed by urea infusion period. In the urea infusion period, urea was continuously infused into the rumen (5.7 g/h) after dissolving in water. Urea infusion increased ruminal ammonia-N from 4.8 to 13.7 mg/dl (P<0.05). Although solid fraction consistently exhibited much greater activities of CMCase and xylanase than liquid fraction (P<0.05), response to urea infusion was somewhat different between these two fractions. Activities of fibrolytic enzyme (CMCase and xylanase) in the rumen digesta were increased by urea infusion, and then the extent of the increase was higher for liquid than for solid fractions. Maximum values for liquid-associated xylanase occurred at 12 h after feeding, while those for the solid-associated occurred at 24 h (P<0.05). Although urea infusion decreased diaminopimelic acid (DAPA) concentration in solid fraction of the ventral digesta, relative abundance of Fibrobacter succinogenes in the ventral digesta tended to increase when urea was infused. Regardless of urea infusion, activities of fibrolytic enzyme from solid fractions were higher in ventral sac than dorsal sac, while opposite results were obtained in liquid fractions. These results indicate that increased ruminal ammonia-N by urea infusion could enhance fibrolytic enzyme activity of liquid-associated microbes at an early stage after feeding, and those of particle-associated microbes at a later stage, through stimulating growth of cellulolytic microbes.