EFFECTS OF FORAGE SPECIES AND PARTICLE-SIZE ON BACTERIAL CELLULOLYTICACTIVITY AND COLONIZATION INSITU

Two ruminally cannulated Holstein heifers were used to evaluate effect s of three forage species and two particle sizes in a 3 x 2 factorial arrangement on in situ fiber disappearance and carboxymethylcellulase activity (CMCase). A second objective was to compare CMCase and N-15 a s relative bacterial colonization markers. At time zero, N-15-enriched ammonium sulfate was dosed into the rumen. Residues in nylon bags inc ubated for 3, 6, 9, 12, 18, 24, 30, 36, 42, 48, 60, and 72 h were anal yzed for NDF, ADF, CMCase, and N-15. Rates of NDF and ADF disappearanc e were faster (P < .05) for red clover (R C) than for gamagrass (GG) o r orchardgrass (OG) and for forages ground to pass a 2-mm screen (P < .05) than for those ground to pass a 5-mm screen. Cumulative area unde r the curve (AUC) of CMCase activity was related to both NDF disappear ance (r2 = .93) and N-15 AUC (r2 = .99), indicating that CMCase can be used to estimate relative colonization of cellulolytic particle-assoc iated bacteria (PAB). During the first 18 h of incubation, RC had grea ter (P < .05) CMCase and N-15 enrichment of in situ residues than did OG or GG. The RC residues also had a greater (P < .05) ratio of CMCase activity:N-15 enrichment; this was presumed to indicate that, during this time, RC supported a greater total PAB population and had a great er proportion of cellulolytic PAB than did either GG or OG. Difference s in the colonization rate of forage particles by cellulolytic PAB rel ative to total PAB may account for differences in rate of fiber digest ion.