Methane emissions status and nutritional strategies for mitigating rumen-produced methane

Methane, the second largest greenhouse gas, is massively attributed to livestock sector globally. Methane from the livestock sector is mainly generated by enteric fermentation of ruminants. With increasing a global demand for beef and milk products, methane emissions are expected to increase, which can accelerate global warming. Methane is a greenhouse gas that has a 28-fold greater global warming potential and a much shorter lifetime than carbon dioxide. Thus, reducing methane emissions from ruminant can mitigate global warming in a short term. There are diverse strategies that have been studied to reduce emissions of rumen-produced methane, including nutritional modifications, genetic selection, and ruminal microbiome manipulations. As nutritional modifications are expected to have quick effects with relatively easy applications, there is a variety of investigations and commercialization on nutritional modifications to reduce methane. In this review, nutritional strategies for mitigating rumen-produced methane are selected if showing more than 10% methane reduction efficacy, and summarized based on their modes of action in the rumen. Nutritional strategies selected in this review can be classified into 3 broad categories; 1) H2 sinks for blocking hydrogenotrophic methanogenesis, 2) methyl-coenzyme M reductase inhibitors in a methane-generating pathway, and 3) eliminators of ruminal methanogens. Propionate, produced by non-fiber carbohydrate in the rumen, and nitrate fed as a feed additive can act as H2 sinks in the rumen. Dietary 3-nitrooxypropanol and red seaweed (Asparagopsis) can inhibit an activity of methyl-coenzyme M reductase. Dietary plant essential oils and unsaturated fatty acids can eliminate ruminal methanogens by toxicity, destructing membranes or inhibiting growth. These nutritional strategies are promising based on sufficient research results and thus need to be applied to reduce emissions of rumen-produced methane. However, a careful application is required for the nutritional strategies as metabolic diseases, negative animal performance, or animal health issues can occur with inadvertent use. Also, some ingredients and feed additives require feeding trials over a very long period to ensure their persistent safety on animals. Therefore, more intensive researches are needed to prove their safe and to develop effective applications in a long term for mitigating rumen-produced methane.