Effect of ruminal cellulolytic bacterial concentrations on in situ digestion of forage cellulose

To evaluate the effects of ruminal cellulolytic bacterial concentrations on in vivo cellulose digestion, varying percentages of flaked soybean hulls were substituted for orchardgrass hay in high-forage diets fed to sheep. In two experiments, total and cellulolytic ruminal bacterial concentrations were not affected by diet. No differences were found for in situ digestion of forage cellulose in the first experiment; however, in Exp. 2, ruminal pH and in situ cellulose digestion were lower (P<.01) with a 40% soybean hull diet. In Exp. 3 with four sheep, two diets were compared, one containing 19.6% cellulose from alfalfa meal and the other 64.3% purified wood cellulose. Ruminal pH was lower (P<.02), 9 and 24 h after feeding, for the high-cellulose diet. Total bacterial concentrations did not change with diet; however, the concentration of cellulolytic bacteria increased (P<.05) when the higher cellulose diet was fed. In situ cellulose digestion was not different between diets. In Exp. 4, 3% sodium bicarbonate was added to the high-cellulose diet, and it was fed twice a day. No differences were observed in pH between diets (P>.42). However, the concentration of total ruminal bacteria increased (P<.06), the concentration of cellulolytic bacteria increased (P<.03), and the percentage of cellulolytic bacteria increased (P<.04) when the buffered high-cellulose diet was fed. In situ digestion of alfalfa cellulose at 30 h was not different between diets (P>.60). These data indicate that the concentration of cellulolytic bacteria is not the limiting factor in the digestion of cellulose in the rumen.