Rumen methanogens, and inhibition of methanogenesis

Methane-producing archaea (known as methanogens) are a distinct group of organisms which are a normal component of the rumen microbial ecosystem. Hydrogen and carbon dioxide are the principal substrates used by rumen methanogens to produce methane (CH4). Because hydrogen and formate are products of fermentation by other microorganisms in the rumen, inhibition of fermentation by other members of the rumen microbial population may in turn inhibit methanogenesis. As well, compounds that inhibit the activity of methanogens directly are likely to reduce or eliminate CH4 production. A strong inverse relationship between the molar proportion of propionate and CH4 production is predicted from knowledge of the interactions among microbial populations in the rumen, and compounds that promote greater production of propionate in the rumen may also have the effect of decreasing CH4 production. Although a wide range of ionophores, antibiotics, and other compounds have been evaluated in vivo and in vitro for their influence on rumen microbial populations and on propionate production, re-evaluation of some of these has been prompted by a search for compounds that both enhance propionate production and decrease production. Where there is a focus on CH4 production by livestock, interest necessarily is in the total amount of CH4 produced per day as a proportion of gross energy intake (%GEI) and its relationship with animal productivity. Because enhanced production of propionate in the rumen also can be associated with an increase in the flow of microbial protein from the rumen, evaluation or re-evaluation of compounds that may be effective in reducing methane production should also include evaluation of the effects on animal productivity, and appropriate approaches are discussed.