Mitigating enteric methane emissions: An overview of methanogenesis, inhibitors and future prospects.

Enteric methane emissions account for approximately 17% of global anthropogenic greenhouse gas emissions and represent 2% to 12% of energy losses from energy intake in ruminants. To reduce these emissions and accelerate the achievement of carbon neutrality, it is critical to understand the factors driving methanogenesis in the rumen and develop effective methane mitigation strategies. Methanogenesis inhibitors, when used in conjunction with nutritional and breeding management strategies, are widely regarded as effective additives for optimizing rumen function, enhancing nutrient utilization and reducing enteric methane emissions. The field of inhibitor development is evolving rapidly under increasing mitigation pressure, necessitating continual review to guide the understanding of their mechanisms of action, effectiveness, risk and potential for widespread use in ruminant production systems. This review compiles data from 78 peer-reviewed in vivo studies conducted over the past 5 years, focusing on 10 inhibitors, which demonstrates 5% to 75% in daily methane emission reduction, 2% to 70% in methane yield reduction, and 11% to 74% in methane intensity reduction. Among the inhibitors, macroalgae are the most effective, achieving 22% to 75% of methane reductions, followed by small targeted molecule inhibitors 3-nitrooxypropanol (3-NOP) with 13% to 62% of methane reductions. Additionally, this review discusses the mechanisms underlying these mitigation strategies, their impact on animal productivity performance, the barriers to their widespread adoption, and directions for future research. Special attention is given to the effects of these inhibitors on rumen hydrogen partial pressure and other metabolic pathways, as improper use may adversely affect nutrient utilization, overall metabolism and animal performance. Future mitigation efforts should focus on the developing next-generation inhibitors that precisely target methanogenic archaea and the methanogenesis pathway. These novel inhibitors must meet on a principle of safety for the host animals, human health and environment, and be economically viable and technically supported with efficiency in achieving long-term mitigation with minimal lifecycle carbon footprints.