Submitted to: Journal Dairy Science Supplement
Publication Type: Abstract Only
Publication Acceptance Date: April 14, 2008
Publication Date: July 7, 2008
Citation: Weimer, P.J. 2008. Factors affecting activity of cellulolytic microbes in the rumen [abstract]. Journal of Dairy Science E-Supplement 1. 91:115. Technical Abstract: Ruminant diets that contain high levels of readily fermentable carbohydrates often display reduced fiber digestibility. However, separating direct from indirect effects has proven difficult. Laboratory culture studies have revealed that the most widely studied ruminal fibrolytic bacteria cannot use starches, and while often capable of using simple sugars, cannot compete effectively for these with typical saccharolytic ruminal bacteria. Glucose and cellobiose are known to inhibit cellulase enzymes in vitro, but their effect in vivo appears to be reduced by their low concentrations, and by the fact that polysaccharide hydrolase enzyme complexes of the fibrolytic bacteria are cell-bound and somewhat protected from the bulk rumen liquid phase. At high sugar concentrations in vitro, rapid sugar fermentation appears to lengthen lag time before initiation of fiber digestion, without necessarily reducing digestion rate. Low ruminal pH (<5.9) inhibits growth of the most well-studied ruminal fibrolytic bacteria, but studies with mixed ruminal microflora in vitro suggest that cellulose degradation continues at substantially lower pH, apparently due to cellulose hydrolysis by cellulase complexes of nongrowing fibrolytic bacteria, coupled with rapid consumption of the released sugars by more acid-tolerant, noncellulolytic bacteria. Cellulose hydrolysis decreases rapidly at pH values < 5.3, apparently due to loss of adherence to cellulose fibers by the fibrolytic bacteria. Unlike starches, pectin does not appear to reduce fiber digestibility, in part, because it is degradable by some fibrolytic bacteria and because its fermentation does not result in dramatic declines in ruminal pH. Available data suggest that complex interactions among the microflora modulate the effects of concentrates on fiber digestion. Moreover, recently developed molecular techniques for characterizing bacterial populations suggest that most ruminal bacterial species have eluded isolation in pure culture, introducing the possibility that additional species may contribute to ruminal fiber digestion.