Strategies to mitigate enteric methane emissions by ruminants - a way to approach the 2.0°C target

Ruminant livestock enteric fermentation contributes approximately one-third of the global anthropogenic methane (CH 4 ) emissions and is projected to increase significantly to meet the increasing demand for animal-sourced protein. Methane, a short-lived greenhouse gas, needs to be reduced -24 to -47% by 2050 relative to 2010 to meet the 2.0°C target. This study describes the results of a comprehensive meta-analysis to determine effective mitigation strategies. The database included findings from 425 peer-reviewed studies (1963 to 2018). Mitigation strategies were classified into three main categories [animal and feed management, diet formulation, and rumen manipulation (additives and methods used to modify the rumen)] and up to five subcategories (98 total mitigation strategy combinations). A random-effects meta-analysis weighted by inverse variance was carried out (Comprehensive Meta-Analysis, V3.3.070). Five feeding strategies, namely CH 4 inhibitors, oils and fats, oilseeds, electron sinks, and tanniferous forages, decreased absolute CH 4 emissions by on average -21% (range -12 to -35%) and CH 4 emissions per unit of product (CH 4 I; meat or milk) by on average -17% (range -12 to -32%) without negatively affecting animal production (weight gain or milk yield). Furthermore, three strategies, namely decreasing dietary forage-to-concentrate ratio, increasing feeding level, and decreasing grass maturity, decreased CH 4 I by on average -12% (range -9 to -17%) and increased animal production by on average 45% (range 9 to 162%). The latter strategies are central to meeting the increasing demand for animal-sourced food. All strategies, but CH 4 inhibitors, can be implemented now and offer immediate approaches for combating global warming.