Modern nutritional approaches to mitigating enteric methane emissions in ruminants: A review

Sustainable livestock production increased public concern towards reducing greenhouse gases. The emission of enteric methane from ruminants, if not well managed, contributes to a significant loss of energy consumed and a marked environmental challenge. Methane has <80 times higher global warming potential than CO2. Enteric methane contributes by <40% of agricultural emissions and 13-19% of global methane emissions. Furthermore, the increased demand for animal protein sources, especially in developing countries, highlights the pressure on greenhouse gas emissions. Therefore, the search for a methane mitigating strategy is a continuous series and a hot topic in animal nutrition laboratories. There are several mitigating nutritional strategies with varying mitigating potentials from 10 to 90% associated with or without digestion or growth complications. These solutions include the use of new ingredients such as insect meal, modulating the quality of food, the inclusion of lipids, and increased concentrate: roughage ration, which decreased CH4 approximately by 18%, 30%, 6% and 50% in the best situations. Furthermore, feed additives have considerable effects on CH4 emissions by inhibiting methanogenesis or competing with substrates for methanogens, such as nanoengineered or nanoencapsulated materials, CH4 inhibitors (statins, bromochloromethan, and 3-NOP) that could inhibit CH4 production by 90%. Several phytobiotic feed additives, including tannins, saponins, and essential oils, have moderate CH4 inhibition effect up to 30%. Seaweed was reported to have nearly complete inhibition of CH4 of the Asparagopsis, Ascophyllum, Dictyota and Rhodophyta genes in vitro or in vivo evaluation. However, the application of these solutions still faces some barriers, including cost, availability, efficacy across production systems, and potential impacts on animal productivity. This review evaluates the mechanisms, effectiveness, and nutritional implications of these nutritional strategies, with particular attention to their applicability in different ruminant production systems (in vivo and in vitro models) for more sustainable production.