Predicting ruminal methane inhibition by condensed tannins
Tannins represent one of the most abundant polyphenolic compounds, second only to lignin. Tannins exist in nature as either hydrolysable tannins or condensed tannins (CT). Hydrolysable tannins are esters of gallic or ellagic acid linked to a sugar core, usually glucose. Condensed tannins, or proanthocyanidins, are polyhydroxy-flavan-3-ol oligomers that have been defined as astringent, high molecular weight polyphenolic compounds that characteristically bind and precipitate proteins. Methane (CH4) is a potent greenhouse gas that is produced normally during microbial fermentation in the rumen and released to the environment during eructation. Methanogens in the rumen digest cellulose into forms usable by the animal. During this process, enteric CH4 is produced during the disposal of metabolic hydrogen. Reducing equivalents that are not consumed during the formation of volatile fatty acids may be used to produce CH4, representing a loss of metabolizable energy to the animal. Condensed tannins may reduce CH4 production by ruminants, though the mechanism of action of CT on methanogenesis is not completely understood. One hypothesis is that CT may directly inhibit the growth of methanogens in the rumen. Another hypothesis is that indirect inhibition occurs by decreasing the availability of nutrients to rumen microorganisms, subsequently reducing substrate digestibility. A decrease in acetate:propionate resulting from an increased transfer of hydrogen to propionate may also inhibit CH4 production when CT are consumed. Predicting ruminal CH4 production is important for ruminant nutrition, especially for the estimation of ME intake to assess the total GE available for metabolism by an animal. Equations have been developed to predict ruminal CH4 production based on dietary constituents. The following nonlinear exponential decay regression equation was developed to predict CH4 inhibition by CT: