Methane production by ruminants: its contribution to global warming

The aim of this paper is to review the role of methane in the global warmin g scenario and to examine the contribution to atmospheric methane made by e nteric fermentation, mainly by ruminants. Agricultural emissions of methane in the EU-15 have recently been estimated at 10.2 million tonnes per year and represent the greatest source. Of these, approximately two-thirds come from enteric fermentation and one-third from livestock manure. Fermentation of feeds in the rumen is the largest source of methane from enteric fermen tation and this paper considers in detail the reasons for, and the conseque nces of, the fact that the molar percentage of the different volatile fatty acids produced during fermentation influences the production of methane in the rumen. Acetate and butyrate promote methane production while propionat e formation can be considered as a competitive pathway for hydrogen use in the rumen. The many alternative approaches to reducing methane are consider ed, both in terms of reduction per animal and reduction per unit of animal product. It was concluded that the most promising areas for future research for reducing methanogenesis are the development of new products/delivery s ystems for anti-methanogenic compounds or alternative electron accepters in the rumen and reduction in protozoal numbers in the rumen. It is also stre ssed that the reason ruminants are so important to mankind is that much of the world's biomass is rich in fibre. They can convert this into high quali ty protein sources (i.e. meat and milk) for human consumption and this will need to be balanced against the concomitant production of methane.