The current study aimed to evaluate if the effect of Saccharomyces cerevisiae (SC) on in vitro fermentation can be affected with the crude protein (CP) content of the ration. Three commercial SC cultures of Biocell F53®, Procreatin 7®, and Biosaf SC47® were evaluated at 0 (SC0), 2 (SC2), and 4 (SC4) mg/g dry matter (DM) of substrate. Two rations with 13% (low crude protein [LCP]) and 16% CP (high crude protein [HCP]) were used as substrates. Rumen gas (gas production [GP]) and methane (CH 4 )

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... oductions were recorded. The HCP ration had increased (P = .05) asymptotic GP, CH 4 production, and fermentation parameters. Biocell F53® and Biosaf SC47® increased the asymptotic GP (P < .05) in HCP and LCP rations with better effect for the dose of 2 mg/g DM substrate HCP (P < .05) and dose of 4 mg yeast/g DM substrate with the LCP ration. The highest CH 4 production was observed (P < .05) with Procreatin 7®. It could be concluded that HCP ration improved GP than LCP ration. Moreover, addition of Biocell F53® and Biosaf SC47® at rate of 2 mg/g DM improved fermentation kinetics and nutrients degradability. Abbreviations: b: the asymptotic gas production; c: the rate of gas production; CH 4 : methane; GY 24 : gas yield at 24 h of incubation; L: the initial delay before gas production begins; MCP: microbial CP production; PF 24 : partitioning factor at 24 h of incubation. ARTICLE HISTORY