Ruminants possess a wide variety of gut microbes that play important roles in digesting fibrous plant materials. Many bioactive compounds，discovered all over the world，have been investigated for effects that would improve ruminant growth and productivity. We investigated the effect of flavonoids contained in buckwheat hull extract on rumen fermentation using an in vitro culture model. Acetate and total short-chain fatty acid production were significantly increased. n-Butyrate production also tended to increase with buckwheat hull extract supplementation. The relative abundance of unclassified Bacteroidales，Clostridiales，and Ruminococcaceae increased，and members of the genus Butyrivibrio displayed an increasing trend with supplementation. Our results suggest that buckwheat hull extract may be a candidate modulator of rumen fermentation. Methane mitigation strategies can confer dual benefits，mitigating greenhouse gas production，and increasing livestock production efficiency. We conducted a preliminary short-term in vitro trial using a garlic and citrus extracts complex，a novel natural feed supplement，to evaluate its effects on rumen fermentation characteristics，methane production，and the bacterial and archaeal community. This extract complex reduced methane production in a dose-dependent manner. We also observed an increase in the relative abundance of propionate-producing bacteria，including Prevotellaceae and Veillonellaceae using garlic-citrus extracts. Among the archaeal community，Methanobacteriaceae were decreased，while Methanomassiliicoccaceae were increased. These results suggest that garlic-citrus extracts are promising natural mixtures for reducing methane production by ruminants. Key Words: rumen，feed additives，microbiome，flavonoids，methane emission
