Abstract: As methane emission is one of the major factors driving global warming and climate change, its reduction from ruminant farming has attracted widespread attention. This research was carried out to evaluate the effect of different levels [0, 0.05%, 0.10% and 0.20%, w/v (weight/volume)] of tannic acid, p-hydroxybenzoic acid, ellagic acid, syringic acid and rutin on ruminal fermentation and methane production in vitro. A mixture of concentrate and forage (65∶35) was used as the substrate. The results showed that adding 0.05% and more tannin significantly reduced the level of ammonia nitrogen (NH₃-N) (P<0.05). Adding 0.20% tannin significantly reduced the fermentation end point pH (P<0.05) and significantly increased the cumulative gas yield at 48 h and the C₂∶C₃ ratio (P<0.05). Adding 0.10% or more p-hydroxybenzoic acid significantly reduced the concentration of NH₃-N and acetate and increased the cumulative gas yield (P<0.05). There was no significant effect of p-hydroxybenzoic acid on the production of volatile fatty acids, with the exception of acetate (P>0.05). When ellagic acid was added, CH₄ production and the C₂∶C₃ ratio significantly reduced (P<0.05). When the addition level of ellagic acid reached 0.10%, TVFA (total volatile fatty acids) decreased significantly (P<0.05), and when the addition level reached 0.20%, pH and NH₃-N also decreased significantly (P<0.05). After the addition of syringic acid, NH₃-N and cumulative gas yield decreased significantly (P<0.05), and when the addition level reached 0.10%, CH₄ and TVFA increased significantly (P<0.05). Rutin significantly decreased NH₃-N and increased TVFA (P<0.05). When the addition level of rutin reached 0.10%, the C₂∶C₃ ratio decreased significantly (P<0.05), and when the addition level reached 0.20%, pH decreased and cumulative gas production increased significantly (P<0.05). In summary, adding a certain level of phenolic acids and rutin can reduce the concentration of NH₃-N and increase gas production. Syringic acid and rutin can increase TVFA production, but ellagic acid reduces TVFA production. Among the additives, only ellagic acid inhibits the production of methane, while syringic acid promotes methane production by ruminal microorganisms.

Key words: phenolic acids, rutin, ellagic acid, in vitro fermentation, methane