Structural Features of Condensed Tannins Influence Their Antimethanogenic Potential in Forage Plants

Despite years of research on the antimethanogenic potential of condensed tannins (CT), their large-scale application is inhibited by a substantial variability in previous studies with regards to their impact on ruminant nutrition. This variability mainly results from the complexity of CT structures, and their impact on methane emissions is often unaccounted for. Hence, this study (a) evaluated the variability in antimethanogenic potential across six forage species, (b) linked methane emissions to tannin activity, and (c) determined the impact of CT structural features on methane abatement. Six forage species were grown in a greenhouse under controlled environmental conditions, namely, sainfoin (Onobrychis viciifolia), birdsfoot trefoil (Lotus corniculatus), big trefoil (Lotus pedunculatus), plantain (Plantaga lanceolata), sulla (Hedysarum coronarium) and lucerne (Medicago sativa). The plants were harvested at the flowering stage and leaf samples were analysed for chemical composition, condensed tannin concentration and structural features, before being incubated in rumen fluid for 24 hours. Lucerne was used as negative control (without tannins) and an additional polyethylene glycol (PEG) treatment was included, to inactivate tannins and link any effect on fermentation characteristics to tannin activity only. A strong variability across the species (P\u3c 0.0001) was observed on methane emissions. Sulla had the highest antimethanogenic potential and decreased methane emissions by 47% compared to lucerne. All species rich in CTs decreased both methane and total gas production, yet the PEG treatment did not alter the methane proportion in the total gas produced. In addition to CT concentration (R= -0.78), methane emissions were found to be negatively correlated with the CT structural features, prodelphinidin percentage (R= -0.6) and mean degree of polymerisation (R= -0.57). This study demonstrated that antimethanogenic potential of forages depends on CT concentration as well as on structural features and incorporating them in the studies can efficiently assess their impact on ruminant nutrition