:The e †ect of daily supplementation of 5 g Saccharomyces cerevisiae yeast culture (YC, YEA-SACC 1026), 30 g supernatant from 5 g YC NaHCO 3 , (YCS), 5 g autoclaved YC (YCH) or 5 g c-irradiated YC (YCR) to the diet of bu †alo calves on rumen microbial populations and fermentation pattern was examined. Addition of 30 g increased the rumen pH to the level NaHCO 3 observed with YC group. The pH and the concentrations of total, total viable and cellulolytic bacteria and total volatile fatty acids (VFA) were signiÐcantly higher while that of lactic acid, hexose-unit oligosaccharides and were NH 3 -N signiÐcantly lower in the rumen Ñuid of YC compared with the control group. The e †ect of was 39É5 and 59É5% in decreasing the concentrations of NaHCO 3 lactic acid and hexose-unit oligosaccharides, 48É1, 47É2 and 45É5% in increasing the numbers of total, total viable and cellulolytic bacteria, 50É0 and 58É1% in increasing the concentrations of total VFA and protein and 51É3% in decreasing the concentration of of YC. The corresponding values for YCR addition NH 3 -N in the diet were 38É6, 45É7, 48É5, 44É4, 51É5, 39É1, 48É1 and 46É5%. The e †ect of YCS and YCH was only marginal, but conspicuous up to 2 h after feeding, in changing the above rumen variables when compared with the YC group. The results indicated that contribution of increase in pH in changing the rumen variables was approximately 50% of YC and almost all the stimulatory activity was associated with live yeast cells. Autoclaving of YC destroyed almost all and cirradiation of YC retained about 50% of stimulatory activity of YC. The e †ect of YC on rumen fermentation, which was maximum up to 2 to 4 h after feeding, decreased with time.1998 SCI. ( J Sci Food Agric 77, 407È413 (1998)

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