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Feed additives as a strategic approach to reduce enteric methane production in cattle: modes of action, effectiveness and safety

Increasing consumer concern in greenhouse-gas (GHG) contributions from cattle is pushing the livestock industry to continue to improve their sustainability goals. As populations increase, particularly in low-income countries, the demand for animal-sourced foods will place further pressure to reduce emission intensity. Enteric methane (CH4) production contributes to most of the GHG from livestock; therefore, it is key to mitigating such emissions. Feed additives have primarily been used to increase animal productivity, but advances in understanding the rumen has resulted in their development to mitigate CH4 emissions. The present study reviewed some of the main feed additives with a potential to reduce enteric CH4 emissions, focusing on in vivo studies. Feed additives work by either inhibiting methanogenesis or modifying the rumen environment, such that CH4 production (g/day) is reduced. Feed additives that inhibit methanogenesis or compete with substrate for methanogens include 3-nitroxypropanol (3NOP), nitrates, and halogenated compounds containing organisms such as macroalgae. Although 3NOP and macroalgae affect methyl–coenzyme M reductase enzyme that is necessary in CH4 biosynthesis, the former is more specific to methanogens. In contrast, nitrates reduce CH4 emissions by competing with methanogens for hydrogen. However, nitrite could accumulate in blood and be toxic to ruminants. Rumen modifiers do not act directly on methanogens but rather on the conditions that promote methanogenesis. These feed additives include lipids, plant secondary compounds and essential oils. The efficacy of lipids has been studied extensively, and although supplementation with medium-chain and polyunsaturated fatty acids has shown substantial reduction in enteric CH4 production, the results have been variable. Similarly, secondary plant compounds and essential oils have shown inconsistent results, ranging from substantial reduction to modest increase in enteric CH4 emissions. Due to continued interest in this area, research is expected to accelerate in developing feed additives that can provide options in mitigating enteric CH4 emissions.