Rumen methane production and its reduction using nutritional factors (review)

Methane is a powerful greenhouse gas with a higher global warming potential than carbon dioxide. Agriculture, especially animal husbandry, is considered the largest sector of anthropogenic methane production. Of farm animals, ruminants are the main producers of methane. Its world production and emissions are increasing due to abundant population of ruminants. The hydrogenotrophic scenario of methanogenesis from hydrogen and carbon dioxide, carried out by ruminal archaea, prevails. Over the past 50 years, numerous research papers have substantially improved our understanding of rumen fermentation and methanogenesis to develope strategies for assessing and reducing methan emission (K.A. Beauchemin et al., 2020). One of the proposed strategies is dietary intervention, i.e. improved dietes and the use of nutritional factors that affect the ruminal microbiota. The quality, feed preparation, the ratio of concentrated and roughage feeds affect methane emissions. Some feeds may increase propionate production or decrease acetate production by reducing the level of ruminal hydrogen converted to methane. Another strategy is the use of modifiers, the feed additives that directly or indirectly inhibit methanogenesis, and biocontrol manipulation using defaunization agents, bacteriocins, bacteriophages, and immunization aimed at reducing the counts of methanogens. The strategy may be also based on genetically or technologically improved productivity performance. With higher productivity, the relative methane emission per unit of meat or dairy product is reduced (M. Islam et al., 2019). Fat additives, organic acids, probiotics, ionophores, phytogenics can serve as strategies to reduce methane formation in ruminants (M. Wanapat et al., 2021; R.D. Marques et al., 2021; S.H. Kim et al., 2020). Feeding manipulation is a simplistic and pragmatic approach to improve animal productivity with a reduced CH4 emission (M.D. Najmul et al., 2018). In the review, along with a description of methanogenesis, we also summaraized modern research data on the influence of various alimentary factors (i.e., special diets, phytogenic saponins, tannins, flavonoids and essential oils) on CH4 emission. The type of diet, the quality of bulky and concentrated feeds, their chemical composition, ratio, pre-feeding preparation affect methane emission in ruminants. However, a promising approach to mitigate methane emissions is adding a small amount of grain to roughage and feeding high quality forages with less fiber and higher levels of soluble carbohydrates. Phytogenics made from various botanical parts of plants is a cheap and environmentally friendly agents to reduce greenhouse gas emissions. Phytogenics also positively affect animal resistance. There are few studies on the in vitro efficacy of flavonoids and other secondary plant metabolites as agents for reducing methane emissions. The data obtained are variable and depend on the type of herbal preparations, their characteristics and the diet fed to the animals. Further in vivo studies should establish the optimal dosages of phytogenics that provide a positive effect. The combination of various phytogenics seems to be relevant and promising. An integrated approach should provide high fragmentation activity, effective digestion and assimilation of feed nutrients.