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

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Abstract

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.

Introduction

Although rumen microbes can utilize preformed amino acids and peptides (Russell et al., 1991), in the case of forage-based diet, most microbial N is derived from the rumen ammonia pool (Hennessy and Nolan, 1988). Studies have been conducted to determine the effects of maximal level on fiber digestion, although the optimum ammonia level for maximal fiber digestion remains unclear (Satter and Slyter, 1974, Mehrez et al., 1977, Erdman et al., 1986, Rihani et al., 1993, Oosting and Waanders, 1993, Dixon, 1999). Also, only limited information is available about the effect of rumen ammonia level on fibrolytic enzyme activity, rumen microbial mass and their distributions in vivo. The information regarding these parameters is useful for optimizing rumen fiber digestion and designing roughage-based feeding systems.

Particle-associated bacteria show lower N concentration (Merry and McAllan, 1983) and less uptake of ammonia-N from rumen ammonia pool, compared to non-associated bacteria (Carro and Miller, 1999). Thus, ruminal liquid- and particle-associated bacteria may show different responses to additional ammonia supply in the rumen, possibly altering their number and fibrolytic enzyme production.

In this study, the ruminal distribution of fibrolytic enzyme (CMCase and xylanase) activities with continuous urea infusion was determined by using steers fed timothy hay, in order to evaluate the contribution of solid- and liquid-associated microbes and their enzymes to fiber digestion and to assess these variations with ammonia supply.

Section snippets

Animal, feeding and sample collection

Three ruminally fistulated steers (average body weight, 625 kg) were fed timothy hay (8.8 kg DM) containing 73.9% neutral detergent fiber (NDF) and 7.0% crude protein (% in DM basis) once daily at 08:30.

The experiment was carried out in two periods, each consisting of 3 weeks adaptation followed by 2 days collection of ruminal samples. In period 1, steers were fed hay only (non-infusion), while in the period 2, urea (reagent grade, Wako, Osaka) solubilized in water (46 g/l) was continuously

Results

The percentages of dry matter in dorsal and ventral rumen digesta showed no apparent diurnal changes after feeding in both non-infusion and urea infusion periods. The values for dorsal sac were not affected by urea infusion (15.1 versus 15.8%, P>0.05), while those for ventral sac were increased from 3.9 to 5.6% with urea infusion (P<0.05).

Fig. 2 presents the concentrations of ammonia-N and total VFA, and pH in the rumen fluid. Urea infusion caused a significantly higher level of ruminal

Discussion

Since the ventral digesta generally contain smaller feed particles, the increased DM proportion of the ventral digesta with urea infusion may suggest the accelerated particle size reduction of the timothy hay as was found by Suzuki et al. (unpublished data), in which urea infusion also enhanced NDF digestibility of the hay. The present results indicate that with urea infusion activities of CMCase and xylanase were significantly elevated, though these changes were influenced by sample fraction

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