The current study was carried out to determine the effect of secondary metabolites on in vitro rumen microbial fermentation and their potential to inhibit methane (CH 4 ) production involved in the global warming and greenhouse-gases (GHG). The obtained results showed that the addition of Yucca schidigera and Quillaja saponaria reduced significantly CH 4 production (P <0.05). This reduction is proportional to the concentration and it becomes superior to 60 % beyond 8 mg / mL of saponins. CH 4

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... oduction and total volatile fatty acid (tVFA) concentration decreased linearly (P<0.05) with increasing dose of essential oils (EO). T he effect was more pronounced with Mentha pulegium. The incorporation of Acacia cyanophylla tannins source to vetch-oat hay leads to a relative decrease in CH 4 in the gas pool. Supplementation with 60% and 30% of Acacia cyanophylla resulted in 37.5 % and 56.25 % lower CH4, respectively. Addition of Acacia cyanophylla reduced significantly tVFA. The proportion of 60% of Acacia cyanophylla induced the highest molar proportion in propionate. The results showed that plant secondary metabolites can be used as feed additives to reduce CH 4 production and to consequently mitigate greenhouse-gases emission.