Can enteric methane emissions from ruminants be lowered without lowering their production?

Methane emissions from ruminant livestock are a contributor to total global anthropogenic emissions of greenhouse gases. We review the most promising dietary and farm system strategies to mitigate enteric CH4 emissions from ruminants, and their potential effects on animal production. Dietary supplementation with fat is the most promising dietary strategy, but the milk production and composition response to supplementary fat is complex and differs among diets. It is also affected by stage of lactation, degree of saturation of the added fat, amount of fat added, and the fat content and composition of the basal diet. To study effects of adding fat to diets on CH4 emissions, a meta-analysis using data from 27 studies was conducted. For diets containing up to 130g fat/kg of dry matter (DM), there was a linear relationship between total fat content of the diet and CH4 yield (g/kg DM intake). The analysis re-run restricting diets to a practical feeding range of <80g fat/kg DM, revealed a difference (P<0.001) between cattle (i.e., dairy and beef) and sheep in their CH4 response to dietary fat. For cattle, a 10g/kg increase in dietary fat decreased CH4 yield by 1g/kg DM intake, but for sheep the decrease was 2.6g/kg, although the relationship for sheep was less precise due to less data (i.e., n=59 for cattle and n=17 for sheep). In the practical range of fat feeding, the relationship between concentration of fat in the diet and CH4 yield was not affected by form of added fat (i.e., oil versus seed), major fatty acids in the added fat (i.e., C12:0 and C:14, C18:1, C18:2, and C18:3), or fat source (i.e., canola, coconut, fatty acid, linseed, soya, sunflower, the basal diet without added fat). Data are also presented which show persistence of the reduction in CH4 emissions as a result of fat supplementation. An update on other dietary strategies such as higher starch diets, use of monensin, exogenous enzymes and use of direct-fed microbials is also provided. Recent studies of dairy and beef farming systems which investigated effects of management strategies on CH4 emissions and livestock production (i.e., milk and beef) using modelling approaches and life cycle assessment are reviewed. Our review demonstrates that dietary and farm management options can be implemented to reduce CH4 emissions from beef and dairy cattle without lowering their production. This article is part of the special issue entitled: Greenhouse Gases in Animal Agriculture - Finding a Balance between Food and Emissions, Guest Edited by T.A. McAllister, Section Guest Editors; K.A. Beauchemin, X. Hao, S. McGinn and Editor for Animal Feed Science and Technology, P.H. Robinson. © 2011 Elsevier B.V.