Harnessing methane proxies to understand and mitigate enteric emissions from ruminant production systems

Methane emissions from livestock, particularly ruminants, significantly contribute to global warming, necessitating the development of accurate methane monitoring systems. Direct methane measurement is technically complex, time-consuming, labour-intensive, and costly. Recent advances in methane inhibitors, such as 3-nitrooxy propanol and halogenated analogues, plant secondary compounds, including polyphenols and essential oils, to reduce methane emissions have necessitated the discovery of processes underlying rumen methane synthesis and inhibition. The identification of methane proxies, such as behavioural and input proxies (dry matter intake, neutral detergent fibre), microbial community proxies (rumen metagenome profiles), metabolic pathway proxies (fatty acids), molecular and genetic proxies (microbial genes), and downstream and non-invasive proxies (milk fatty acids and faecal lipidomes), is leading to more viable solutions. New developments in 'omic' techniques, including lipidomics, metagenomics and metatranscriptomics, have enabled the detection of proxies at the molecular level utilising rumen liquor, milk, blood, urine, and faeces. In addition to traditional methane proxies, rumen microbiota profiles, and specific genes involved in rumen methanogenesis (such as mcr and mrt, which encode methyl coenzyme reductase 1 and 2), these markers can be used to identify methane-producing pathways. Protozoa-associated methanogens (PAMs), propionate-producing bacteria, and methane-oxidising methanotrophs (Methylocystis sp.) are emerging as new proxies. Methane proxies provide scalable, affordable, and mechanistically insightful alternatives to conventional direct measuring techniques, which improve the understanding of rumen function and the biological causes of methane releases, enabling large-scale methane monitoring and will enable designing effective methane mitigation strategies in livestock production systems.