Insights into the Feed Additive Inhibitor and Alternative Hydrogen Acceptor Interactions: A Future Direction for Enhanced Methanogenesis Inhibition in Ruminants

Enteric methane (CH4) emissions from ruminants contribute significantly to agricultural greenhouse gases. Anti-methanogenic feed additives (AMFA), such as Asparagopsis spp. and 3-nitrooxypropanol (3-NOP), reduce CH4 emissions by inhibiting methanogenic enzymes. However, CH4 inhibition often leads to dihydrogen (H2) accumulation, which can impact rumen fermentation and decrease dry matter intake (DMI). Recent studies suggest that co-supplementation of CH4 inhibitors with alternative electron acceptors, such as phloroglucinol, fumaric acid, or acrylic acid, can redirect excess H2 during methanogenesis inhibition into fermentation products nutritionally beneficial for the host. This review summarises findings from rumen simulation experiments and in vivo trials that have investigated the effects of combining a CH4 inhibitor with an alternative H2 acceptor to achieve effective inhibition of methanogenesis. These trials demonstrate variable outcomes depending on additive combinations, inclusion rates, and adaptation periods. The use of phloroglucinol in vivo consistently decreased H2 emissions and altered fermentation patterns, promoting acetate production, compared to the co-supplementation with fumaric acid or acrylic acid as an alternative electron acceptor. As a proof of concept, phloroglucinol shows promise as a co-supplement for reducing CH4 and H2 emissions and enhancing volatile fatty acid profiles in vivo. Optimising microbial pathways for H2 utilisation through targeted co-supplementation and microbial adaptation could enhance the sustainability of CH4 mitigation strategies using feed additive inhibitors in ruminants. Further research using multi-omics analyses is needed to understand microbial dynamics and improve the efficacy of CH4 inhibitor interventions with an alternative H2 acceptor co-supplementation in vivo.