Climate-Resilient Agriculture: Systems, Science and Solutions for a Warming World

Enteric fermentation in dairy cattle is a large and policy-relevant source of methane, and the past decade has produced two classes of compound capable of suppressing ruminal methanogenesis far more strongly than conventional dietary manipulation. The first acts by direct inhibition of methyl-coenzyme M reductase, the terminal enzyme of methanogenesis; the second delivers halogenated methane analogues, chiefly bromoform, from red macroalgae of the genus Asparagopsis. Both have moved from proof of concept to commercial or near-commercial deployment more quickly than the supporting evidence has matured, and existing reviews were largely written before the appearance of grazing trials, dose-by-diet interaction studies, residue and animal-health assessments, and full-lactation datasets that now qualify earlier optimism. This critical narrative review evaluates the mechanistic basis, quantitative efficacy, consistency, and translational limits of enteric methane inhibitors in dairy cattle, and distinguishes conclusions that are well supported from those that remain conditional. Evidence was drawn from scholarly databases and indexes searched to a defined end date, appraised for design adequacy, measurement method, exposure duration and confounding, and synthesised thematically around mechanism, efficacy, hydrogen disposal, delivery, safety and system-level translation. The available evidence supports a reproducible reduction of roughly one third in methane production when 3-nitrooxypropanol is delivered continuously in a mixed ration, with efficacy declining as dietary fibre increases and rising with dose. Macroalgal preparations can achieve larger reductions but do so with less consistency, with frequent depression of feed intake, with substantial instability in the concentration of the active compound, and with unresolved residue and animal-health questions. Nitrate occupies an intermediate position constrained by stoichiometry and toxicity. The dominant unresolved problems are not mechanistic but translational: intermittent exposure in grazing systems, the fate of spared metabolic hydrogen, the durability of responses across whole lactations, and the credibility of measurement for emissions accounting. Progress now depends less on discovering new inhibitors than on designing delivery, verification and governance systems adequate to the compounds already available.