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