At present, there is little information regarding whether supplementation with Piper betle powder (PBP) and sunflower oil (SFO) has a synergistic effect on lowering methane emissions without negatively impacting ruminal fermentation. This study investigated the effects of PBP, supplemented either with or without SFO, on biogas release, fermentation end-products, and microorganisms in the rumen of lactating goats. The treatments were run in a completely randomized 3 × 5 factorial arrangement,

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... reby 0, 15, and 30 mg SFO were combined with 0, 15, 30, 45, and 60 mg PBP on a dry matter basis. The outcomes were assessed in vitro. PBP was obtained from the perennial plant Piper betle L., which is an abundant source of flavonoids and their aromatic derivatives. SFO, which reduces dietary methane emissions, was supplemented to confirm whether it interacted with other nutrients in the ruminant diet. SFO × PBP significantly (p < 0.05) decreased methane production, enhanced total volatile fatty acid concentrations, and decreased the number of rumen protozoa. We found that 15-30 mg, but not 45-60 mg, PBP combined with 0, 15, and 30 mg SFO increased (p < 0.05) total gas production (including CO2) from fermentation. However, our results suggested that at least 45 mg PBP, either alone or combined with SFO, was required to reduce ammonia-N (p < 0.05). Not all treatments affected rumen pH. In conclusion, supplementing PBP (< 30 mg), either alone or combined with SFO, has a suppressing effect on methane production while preserving an optimum rate of rumen fermentation.