-The aim of this paper is to review the role of methane in the global warming scenario and to examine the contribution to atmospheric methane made by enteric 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 fermentation and this paper considers in detail the reasons for, and the consequences 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 propionate formation can be considered as a competitive pathway for hydrogen use in the rumen. The many alternative approaches to reducing methane are considered, 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 systems for anti-methanogenic compounds or alternative electron acceptors in the rumen and reduction in protozoal numbers in the rumen. It is also stressed 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 quality protein sources (i.e. meat and milk) for human consumption and this will need to be balanced against the concomitant production of methane. methane / ruminants / global warning / reduction strategies Résumé -Production de méthane par les ruminants : sa contribution au réchauffement de la planète. Cet article examine le rôle du méthane dans le processus de réchauffement de la planète et évalue la contribution au méthane atmosphérique des gaz d'origine digestive issus principalement des ruminants. Les émissions annuelles de méthane d'origine agricole dans l'Europe des quinze ont été estimées récemment à 10,2 millions de tonnes et représentent la principale source des entrées Ann. Zootech. 49 (2000) 231-253 231 © INRA, EDP Sciences

Fifteen potential precursors of propionate were tested for their ability to decrease CH 4 production by ruminal fluid in vitro. Sodium acrylate and sodium fumarate produced the most consistent effects in batch cultures, with 50 % of the added precursors being fermented to propionate and CH 4 production decreasing by between 8 and 17 %, respectively. Additives were more effective when added as free acids, but this also decreased the pH and may have inhibited fibre digestion. Changing the dietary substrate from predominantly grass hay to predominantly concentrate had no influence on the effectiveness of acrylate and fumarate. In an in vitro fermentor (the rumen simulating technique, Rusitec) with a grass hay-concentrate (50:50, w/w) diet as substrate, both compounds were again fermented to propionate (33 and 44 % conversion to propionate, respectively). However, fumarate appeared more effective as a H 2 sink compound. It was calculated to capture 44 % of the H 2 previously used for CH 4 formation compared with a 22 % capture of H 2 with acrylate. Fumarate also caused a stimulation in fibre digestion. Thus, sodium fumarate was the preferred propionate precursor for use as a feed ingredient to decrease CH 4 emissions from ruminants.

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Two experiments were conducted, at ADAS Drayton in the autumn and winter 1996/1997, to compare methane (CH4) emissions from sheep housed either in a polytunnel system or in open-circuit respiration chambers. In each system, the sheep received maintenance levels of either cut grass or high temperature dried grass pellets (HTDG). All experiments in the tunnel were conducted on concrete to avoid any interactions of the CH4 with the soil/plant environment. The results suggested that CH4 production from the open-circuit chambers was greater than from the tunnel system (26·9±0·46 v. 31·7±0·35 l/kg dry matter intake (±S.E.) for open circuit respiration chambers and tunnel, respectively). Recovery tests gave similar results for both systems (95·5–97·9% for tunnels and 89·2–96·7% for chambers), and confirmed that both methods give good quantitative recovery of added CH4, and can therefore be assumed to provide reliable estimates of emissions from animals. There is no technical explanation, therefore, for the different estimates of emissions provided by the two systems. Further studies are required to understand the reasons for the differences and in particular, the possible links between animal behaviour induced by the two systems and CH4 emission rates.