Dietary mitigation of enteric methane from cattle.

Abstract Enteric methane from ruminants accounts for about 11-17% of methane generated globally, or 17-30% of methane from human activity. Methane arises from the activity of methanogens in the rumen that use hydrogen to reduce carbon dioxide, thereby preventing the accumulation of reducing equivalents, which would otherwise impede ruminal fermentation. Although this process is desirable from a fermentation perspective, it is energetically costly, as cattle emit 2-12% of their gross energy intake in this potent greenhouse gas (GHG). Many production practices aimed at increasing efficiency of production, such as including grain and ionophores in diets, also lower methane emissions. These practices were adopted long before issues arose over the role of methane from livestock in climate change. Dietary inclusion of free oils or oil-rich feeds (e.g. oilseeds, distillers' grains and micro-algae), biologically active plant compounds (e.g. condensed tannins, saponins and essential oils), rumen fermentation modifiers (e.g. yeast and bacterial direct-fed microbials), as well as improvements in forage quality may allow for further reductions in methane emissions from cattle. The optimum dietary strategy will depend on the particular farm, its geographic location, the feedstuffs available and the type of animals being fed. Reductions can occur as decreased methane output per animal per day or as decreased methane output per kg of meat or milk produced, but ultimately, it seems prudent that mitigation practices be assessed on the basis of the extent to which they reduce methane emissions per kg of meat or milk produced. Furthermore, potential mitigation practices need to be assessed from the perspective of the entire life cycle, as a reduction in GHG in one sector of the production cycle can often lead to changes in GHG emissions in another sector.