The objective of this study was to evaluate the capacity of the live yeast (LY) Saccharomyces cerevisiae in optimizing ruminal pH and in understanding its mode of action during induced acidosis in dairy cow. Two non-lactating cannulated cows were used and offered twice daily a control diet (CD) consisting of 51% corn silage and 49% concentrates or a LY diet (LYD) composed of CD supplemented with 4 g of LY per cow and per day. Measurements of pH and redox potential (E h ) were continuously made at 1 h interval during an experimental period of 9 h per day. Samples of ruminal fluid were also taken at 2 h intervals for analyses of volatile fatty acids (VFA) and lactic acid. Oxygen partial pressure (log P O 2 ) in the ruminal milieu was calculated from the Nernst equation, using either O 2 -H 2 O or lactate-propionate redox couples. The results showed an increase of 0.2 unit in ruminal pH when LYD was fed, which was accompanied by a mean difference in E h of －20 mV with respect to CD. The log P O 2 decreased significantly by 0.8 log unit for LYD when compared to CD. Concentrations of VFA and proportion of propionate were higher with LYD (114.4 mM and 17.1% total VFA) compared to CD (102.4 mM and 15.4% total VFA). Proportion of butyrate decreased (from 15.8% to 14.2% total VFA). Lactate concentration decreased by 55% on average. It is proposed that the stabilization of ruminal pH (>6) is the outcome of the LY ability to scavenge oxygen. In doing so, it increased the reducing capacity of the milieu favoring the production of total VFA and depressing the level of lactate. The LY oxygen-scavenging ability was put forward to account for the transformation of lactate (pK a = 3.86) into propionate (pK a = 4.87) under reduced ruminal conditions.
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