Review: Methanogens and methane production in the digestive systems of nonruminant farm animals.

The greenhouse gases (GHGs) derived from agriculture include carbon dioxide, nitrous oxide, and methane (CH). Of these GHGs, CH, in particular, constitutes a major component of the GHG emitted by the agricultural sector. Along with environmental concerns, CH emission also leads to losses in gross energy intake with economic implications. While ruminants are considered the main source of CH from agriculture, nonruminant animals also contribute substantially, and the CH emission intensity of nonruminants remains comparable to that of ruminants. Means of mitigating CH emissions from enteric fermentation have therefore been sought. Methane is produced by methanogens-archaeal microorganisms that inhabit the digestive tracts of animals and participate in fermentation processes. As the diversity of methanogen communities is thought to be responsible for the differences in CH production among nonruminant animals, it is necessary to investigate the archaeal composition of specific animal species. Methanogens play an important role in energy metabolism and adipose tissue deposition in animals. Higher abundances of methanogens, along with their higher diversity, have been reported to contribute to lean phenotype in pigs. In particular, a greater abundance of Methanosphaera spp. and early dominance of Methanobrevibacter smithii have been reported to correlate with lower body fat formation in pigs. Besides the contribution of methanogens to the metabolic phenotype of their hosts, CH release reduces the productivity that could be achieved through other hydrogen (H) disposal pathways. Enhanced participation of acetogenesis in H disposal, leading to acetate formation, could be a more favorable direction for animal production and the environment. Better knowledge and understanding of the archaeal communities of the gastrointestinal tract (GIT), including their metabolism and interactions with other microorganisms, would thus allow the development of new strategies for inhibiting methanogens and shifting toward acetogenesis. There are a variety of approaches to inhibiting methanogens and mitigating methanogenesis in ruminants, which can find an application for nonruminants, such as nutritional changes through supplementation with biologically active compounds and management changes. We summarize the available reports and provide a comprehensive review of methanogens living in the GIT of various nonruminants, such as swine, horses, donkeys, rabbits, and poultry. This review will help in a better understanding of the populations and diversity of methanogens and the implications of their presence in nonruminant animals.