The effect of different classes of tannins on methanogenesis in ruminants (review)

The emission of greenhouse gases by ruminants is becoming an urgent environmental problem. Methane has the highest global warming potential among greenhouse gases (R.A. Muller et al., 2017). Reducing gastrointestinal CH4 emissions will help improve energy efficiency and reduce the environmental burden from agriculture (I.V. Petrunina et al., 2022). There are several strategies to reduce greenhouse gas emissions from ruminants, in particular, nutrition management and the use of various feed additives (N.V. Bogolyubova et al., 2022). Various fat additives are also used to reduce methane emissions. Dietary unsaturated fatty acids (FFAs) have a detrimental effect on methanogens and protozoa and reduces the acetate/propionate ratio in the rumen, resulting in reduced methane production (J.O. Zeitz et al., 2013). Another promising strategy to reduce methane emissions is the use of secondary plant metabolites as feed additives. In particular, these are tannins which have anti-methanogenic potential, pronounced antioxidant, antimicrobial properties, and can form complexes with proteins and some trace elements due to the presence of a large amount of phenolic hydroxyl groups (A.I. Roca-Fernández et al., 2020; P.R. Lima et al., 2019). These are high molecular weight polyphenolic compounds of two groups, the condensed and hydrolysable tannins (A.K. Patra et al., 2010). Their biological activity largely depends on the chemical structure and dosage. Hydrolysable tannins at high concentrations have a toxic effect, unlike condensed tannins. Mechanisms of tannins’ action are not fully understood. One hypothesis is that tannins act directly on methanogens in the rumen, changing the membrane permeability of some rumen microorganisms and inhibiting their enzymatic activity (M. Caetano et al., 2019). Another hypothesis is that indirect inhibition occurs due to a decrease in the availability of nutrients for rumen microorganisms, which subsequently reduces the digestibility of the substrate and indirectly inhibits rumen microbial populations (H.D. Naumann et al., 2017). A third hypothesis for how tannins, namely condensed tannins, inhibit CH4 production is that they act as a proton scavenger (H.D. Naumann et al., 2017). To date, a lot of work is underway to study the effect of various classes of tannins on the cicatricial microbiota and methanogenesis. Numerous in vitro and in vivo studies show that the inclusion of tannins directly from plants or as plant extracts in the diet of ruminants leads to a decrease in CH4 emission (F. Hassanat et al., 2013, H.M. El-Zaiat et al., 2020). Some studies also evaluate the effect of a mixture of condensed and hydrolysable tannins on rumen fermentation (C.J. Marshall et al., 2022).