Bigger is not always better: A study of the structure-activity relationship of oligomeric ellagitannins on ruminal fermentation in vitro

Methane from enteric fermentation is a natural by-product of the digestive processes of ruminants which represents a loss of 2 to 12% of the animal's groß energy intake [1]. Plant secondary metabolites and tannins in particular, have shown some promising results as dietary inhibitors of methanogenesis. However, the structure-activity relationships that underlie the mechanisms of action of tannins in the rumen remain mostly unkown. In this study, we investigated the effect of the degree of oligomerization of ellagitannins (ET) on their ability to alter ruminal fermentation. For that purpose we isolated dimeric to heptameric ET from rosebay willowherb (Epilobium angustifolium) and tested these compounds on several indicators of ruminal fermentation with an in vitro gas production system. Results show that oligomeric ET decreased total volatile fatty acids production proportionally to their degree of oligomerization. Methane production was also decreased but the experiment did not reveal any difference in the level of activity of the ET. Additionally, willowherb's oligomeric ET decreased ammonia-nitrogen and branched-chain volatile fatty acids concentrations, thus indicating reduced protein degradation by ruminal bacteria. In conclusion this study shows that the various activities of willowherb's oligomeric ET towards ruminal fermentation are highly dependent on their chemical structure. But more importantly, smaller oligomers had less detrimental effect toward organic matter fermentation than the larger ones while displaying similar level of antimethanogenic activity.

Keywords: Gas production technique, willowherb, methane, structure-activity relationship.

References:

[1] Johnson KA, Johnson DE. Methane emissions from cattle. J Anim Sci 1995; 73: 2483 – 2492