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

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Abstract

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 130 g 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 <80 g 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 10 g/kg increase in dietary fat decreased CH4 yield by 1 g/kg DM intake, but for sheep the decrease was 2.6 g/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.

Introduction

Agriculture was responsible for 10–12% of total global non-CO2 greenhouse gas (GHG) emissions in 2005, but emissions of CH4 and N2O increased globally by nearly 17% from 1990 to 2005, with both gases contributing equally to the increase (Smith et al., 2007). Enteric CH4 fermentation accounted for about 32% of total non-CO2 emissions from agriculture in 2005 (Smith et al., 2007). If CH4 emissions grow in direct proportion to projected increases in livestock numbers, then global CH4 emissions from livestock production are expected to increase 60% by 2030 (FAO, 2003). Efforts are being made by governments around the world to develop mitigations to reduce CH4 emissions from ruminant livestock. However, livestock producers are unlikely to adopt these strategies if they reduce animal production and, hence, profitability.

Given the contribution of CH4 to global GHG production, there have been several recent reviews of mitigation strategies to reduce enteric CH4 emissions from livestock (i.e., Beauchemin et al., 2008, Beauchemin et al., 2009b, Eckard et al., 2010, Martin et al., 2010). In addition to these reviews, other papers in this issue offer in depth evaluation of specific CH4 mitigation options, including use of plant derived essential oils, the potential for animal based options such as improved feed conversion efficiency, identifying high and low CH4 emitting animals and the feasibility of manipulating the rumen microbiota. As a consequence, our review focuses on other promising dietary and farming system strategies with particular emphasis on effects on animal production and CH4 emissions. In addition, emphasis is placed on achieving a net global reduction in potential GHG mitigations and, hence, use of life cycle assessment (LCA) is considered so that all on- and off-farm emissions are included (Weiske et al., 2006).

The potential for dietary supplementation with fat is a promising dietary strategy and is examined in detail for both total mixed rations (TMR) and pasture based grazing systems. An update is presented for other promising strategies including use of high starch forages, monensin, enzyme additives, yeasts and direct fed microbials. Recent studies of dairy and beef farming systems that used modelling approaches and LCA are reviewed. These studies investigated effects of management strategies on CH4 emissions and livestock production (i.e., milk and beef).

Section snippets

Fats – production responses

From a nutritional perspective, Jenkins (1997) categorized fat supplements for dairy rations by how they affect ruminal fermentation and fiber digestion. Calcium salts of fatty acids and hydrogenated fats are designed specifically to avoid problems related to reduced fermentation in the rumen. These fats have little or no negative effects on fiber digestion in the rumen at normal levels of supplementation because they are not released in the rumen. Another group of fats includes unaltered

Farming system strategies

The dietary strategies discussed thus far have aimed at lowering enteric CH4/kg feed consumed without considering the potential for an increase in GHG elsewhere in the production chain. Beauchemin et al. (2009b) and Eckard et al. (2010) stressed benefits of using whole farm systems modelling and LCA in assessing whole farm impacts of any mitigation. For example, improving pasture digestibility may reduce CH4 yield as g/kg DM, but it is also likely to increase DM intake and thus total CH4

Conclusions

This review has shown that addition of fat to the diet can result in a persistent decrease in CH4 emissions, and not lower animal production. The challenge is to identify fat sources that can be feasibly added to the diet in a cost effective manner that also result in a net reduction in GHG emissions (as kg/d and kg/kg of product). Other dietary strategies including higher starch diets, and use of some feed additives can also help lower emissions while improving cattle performance. For a

Conflict of interest statement

None.

Acknowledgements

The authors thank Stefan Muetzel (AgResearch, Palmerston North, New Zealand) and Toby Entz (Agriculture and Agri-Food Canada) for their assistance with the meta-analysis of experiments that added fat to ruminant diets and Sheila Torgunrud (Agriculture and Agri-Food Canada) for preparing the figures.

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