Enteric methane mitigation technologies for ruminant livestock: a synthesis of current research and future directions

Enteric methane (CH4) emission in ruminants, which is produced via fermentation of feeds in the rumen and lower digestive tract by methanogenic archaea, represents a loss of 2% to 12% of gross energy of feeds and contributes to global greenhouse effects. Globally, about 80 million tonnes of CH4 is produced annually from enteric fermentation mainly from ruminants. Therefore, CH4 mitigation strategies in ruminants have focused to obtain economic as well as environmental benefits. Some mitigation options such as chemical inhibitors, defaunation, and ionophores inhibit methanogenesis directly or indirectly in the rumen, but they have not confirmed consistent effects for practical use. A variety of nutritional amendments such as increasing the amount of grains, inclusion of some leguminous forages containing condensed tannins and ionophore compounds in diets, supplementation of low-quality roughages with protein and readily fermentable carbohydrates, and addition of fats show promise for CH4 mitigation. These nutritional amendments also increase the efficiency of feed utilization and, therefore, are most likely to be adopted by farmers. Several new potential technologies such as use of plant secondary metabolites, probiotics and propionate enhancers, stimulation of acetogens, immunization, CH4 oxidation by methylotrophs, and genetic selection of low CH4-producing animals have emerged to decrease CH4 production, but these require extensive research before they can be recommended to livestock producers. The use of bacteriocins, bacteriophages, and development of recombinant vaccines targeting archaeal-specific genes and cell surface proteins may be areas worthy of investigation for CH4 mitigation as well. A combination of different CH4 mitigation strategies should be adopted in farm levels to substantially decrease methane emission from ruminants. Evidently, comprehensive research is needed to explore proven and reliable CH4 mitigation technologies that would be practically feasible and economically viable while improving ruminant production.