Reduction in methane emissions from ruminants by plant secondary metabolites: effects of polyphenols and saponins

The effects of plant secondary metabolites (PSM), specifically polyphenols (tannins) and saponins on rumen fermentation and methnogenesis were investigated using the Hohenheim gas method. We evaluated the effects of: (1) polyphenol-containing plants, (2) simple phenols in the form of phenolic acids, (3) purified tannins, (4) saponin-containing plants, and (5) isolated saponin-rich fractions on rumen methanogenesis. Statistically significant negative relationships between total phenols, total tannins or tannin activity and methane (CH4) production were observed, whereas no correlation existed between condensed tannins and CH4 production. Cinnamic, caffeic, p-coumaric and ferulic acids decreased CH4 production significantly when added at 5 mM. Addition of purified chestnut and sumach tannins (hydrolysable tannins) at 1 mg/mL to the in vitro rumen fermentation system containing hay:concentrate (70:30) decreased CH4 production (P < 0.05), by 6.5 and 7.2% respectively. However, addition of mimosa and quebracho tannins (condensed tannins) at this concentration did not decrease CH4 production. For studying the effects of saponins, leaves of Sesbania, Knautia and seeds of Fenugreek, and their saponin-rich fractions were evaluated. Addition of Fenugreek and Sesbania plant materials to hay or the hay-concentrate mixture increased partitioning factor (PF, expressed as mg truly degraded substrate/mL gas produced; a measure of efficiency of microbial protein synthesis) and decreased CH4 production per unit substrate degraded. These plant materials and their saponin-rich fractions did not reduce CH4 production in absolute amounts despite decreases in protozoal numbers by 40-50%. The saponins altered the microbial community towards proliferation of fibre-degrading bacteria and inhibition of fungal population. The results with saponin-containing plant materials and their isolated fractions indicated a weak association between anti-protozoal activity of saponins and methanogenesis. Nevertheless, the saponin-containing plants possess potential to partition higher proportions of the substrate to microbial mass production. (author)