Structural features of condensed tannins affect in vitro ruminal methane production and fermentation characteristics

An in vitro study was conducted to investigate the effects of condensed tannin (CT) structural properties, i.e. average polymer size (or mean degree of polymerization), percentage of cis flavan-3-ols and percentage of prodelphinidins in CT extracts on methane (CH₄) production and fermentation characteristics. Condensed tannins were extracted from eight plants in order to obtain different CT types: blackcurrant leaves, goat willow leaves, goat willow twigs, pine bark, redcurrant leaves, sainfoin plants, weeping willow catkins and white clover flowers. They were analysed for CT content and CT composition by thiolytic degradation, followed by high performance liquid chromatography (HPLC) analysis. Grass silage was used as a control substrate. Condensed tannins were added to the substrate at a concentration of 40 g/kg, with or without polyethylene glycol (+ or −PEG 6000 treatment) to inactivate tannins, then incubated for 72 h in mixed buffered rumen fluid from three different lactating dairy cows per run. Total cumulative gas production (GP) was measured by an automated GP system. During the incubation, 12 gas samples (10 µl) were collected from each bottle headspace at 0, 2, 4, 6, 8, 12, 24, 30, 36, 48, 56 and 72 h of incubation and analysed for CH₄. A modified Michaelis-Menten model was fitted to the CH₄ concentration patterns and model estimates were used to calculate total cumulative CH₄ production (GPCH₄). Total cumulative GP and GPCH₄ curves were fitted using biphasic and monophasic modified Michaelis-Menten models, respectively. Addition of PEG increased GP, GPCH₄, and CH₄ concentration compared with the −PEG treatment. All CT types reduced GPCH₄ and CH₄ concentration. All CT increased the half time of GP and GPCH₄. Moreover, all CT decreased the maximum rate of fermentation for GPCH₄ and rate of substrate degradation. The correlation between CT structure and GPCH₄ and fermentation characteristics showed that the proportion of prodelphinidins within CT had the largest effect on fermentation characteristics, followed by average polymer size and percentage of cis flavan-3-ols.