this study aimed at determining the chemical composition of sage essential oil (seo) and yar- row essential oil (yeo), and investigate in vitro their impacts on gas production kinetics, ruminal digestibility and fermentation, and rumen methanogenesis at different dosages (0, 250, 500 and 750 mg l –1 for seo; and 0, 250, 500, 750 and 1000 mg l −1 for yeo). alpha-pinene and 1,8 cineol were two major constituents of both seo and yeo. Both seo and yeo had a linear and quad- ratic effect on asymptotic gas production (p<0.05). the gas production rate increased linearly with seo and curve-linearly with yeo dosages (p<0.05). in vitro degradability of dry matter and or- ganic matter decreased only by yeo. the partitioning factor (pf) and the microbial biomass (mB) decreased and increased linearly with yeo and seo dosages, respectively (p<0.05). total volatile fatty acids (Vfa) were not affected by seo, but decreased in a linear and quadratic manner with YEO dosage (P<0.05). The VFA pattern was modified in a linear and quadratic manner by both seo and yeo (p<0.05). ammonia concentration increased linearly only with yeo increasing doses. the methane to total gas (tg) ratio decreased quadratically only by seo with reductions of 6.7, 13 and 4.2% at the doses of 250, 500 and 750 mg l −1 , respectively. these results revealed that SEO modifies the rumen fermentation positively towards producing more MB and less methane in the dose range of 0–750 mg l −1 , however, yeo adversely affected the rumen fermentation at all the tested doses.
