INFLUENCE OF YEAST CULTURE SUPPLEMENTATION AND ADVANCING SEASON ON STEERS GRAZING MIXED-GRASS PRAIRIE IN THE NORTHERN GREAT-PLAINS .2. RUMINAL FERMENTATION, SITE OF DIGESTION, AND MICROBIAL EFFICIENCY

Twelve ruminally and duodenally cannulated beef steers (initial BW 368 +/- 25.3 kg) and four ruminally cannulated beef heifers (initial BW 5 59 +/- 79.5 kg) were used to evaluate the effects of yeast culture (YC ) and advancing season on ruminal fermentation, microbial protein synt hesis, ruminal fluid kinetics, and site of digestion. Treatments were control and YC supplementation (28.4 g.steer(-1).d(-1) dosed ruminally ). Steers grazed from late June to early November 1991 on mixed-grass prairie. Ruminal pH decreased (P < .10) from late July to early Octobe r. Fluid dilution rate decreased (P < .10) as the grazing season advan ced, whereas ruminal fluid volume and flow rate increased (P < .10) fr om late July to early October. Ruminal ammonia concentration (milligra ms/deciliter) was lower (P < .10) during late July and late August tha n during late June and early October at 4, 8, 12, and 16 h after sunri se in YC supplemented steers. Molar proportions of propionate and buty rate were greater (P < .10) in control than in YC-supplemented steers at 0, 4, 12, and 24 h and 0, 4, and 12 h after sunrise, respectively, during late July. Acetate (mol/100 mel) was greater (P < .10) during l ate July and late August than in late June and early October for YC-su pplemented steers at 0, 4, and 8 h after sunrise. True ruminal OM dige stion was greatest (P < .10) in late June, intermediate in late July a nd late August, and least in early October. Supplementation with YC in creased (P < .04) true ruminal OM digestion in late June and late July . Steers receiving YC had greater (P < .07) duodenal bacterial N flow in late July. These data indicate that yeast culture supplementation c an increase true OM digestibility early in the grazing season. Advanci ng season seems to result in increased ruminal fluid volume, lower tru e ruminal OM digestion, and greater microbial efficiency.