Effects of two microbial feed supplements on microbial activities in rumen-stimulating
cultures and the mens of steers fed a fescue hay-based roughage diet were evaluated. The
yeast culture supplement contained Saccharomyces cerevisiue (1.4 to 4.2 x lo9 colonyforming
units [cfu]/g), whereas the mixed microbial supplement contained yeast,
lactobacilli and enterococci (1.4 to 2.7 x 109 cfu/g, 1.2 to 2.3 x lo9 cfu/g, and 1.5 to 2.6 x
1Olo cfu/g, respectively). Concentrations of viable yeast cells were increased consistently in
continuous cultures and rumens of steers receiving either supplement (1 glkg of feed).
However, neither supplement consistently altered the relative concentrations of volatile
fatty acids or ammonia in continuous cultures and mens of steers. The pH tended to be
greater (P = .13) in continuous cultures receiving yeast culture supplement than in cultures
receiving the unsupplemented diet (6.50 vs 6.36), but pH in the rumens of steers was not
affected by the supplements. Concentrations of cellulolytic microorganisms in cultures and
the m e n s of steers receiving supplements containing only yeast were from 5 to 40 times
greater than those observed in cultures or steers receiving the unsupplemented diet.
Supplements that had been treated with heat (121'C for 15 min) to inactive yeast cells did
not alter the concentrations of cellulolytic bacteria in rumen-stimulating cultures. These
results suggest that live yeast culture supplements stimulate growth of cellulolytic
microorganisms in the rumen.
(Key Words: Cattle, Rumen Bacteria, Yeasts, Cellulolytic Microorganisms, Artificial
RUmen.)