Dose-Response Effect of Prairie Acacia Condensed Tannins On Ruminal Methanogenesis: Structure-Activity Relationships.

Interactions between rumen microorganisms and their ruminant host are mediated in part by chemical constituents of the host diet. Understanding how diet affects interactions between ruminants and gastrointestinal microbes could result in the use of novel forages to improve animal productivity and reduce production of the greenhouse gas methane (CH₄), a byproduct of rumen fermentation. We have evaluated Acacia angustissima var. hirta (prairie acacia; PA) for its condensed tannin (CT) characteristics in previous studies and determined there is a moderate amount of bioactive CT (protein precipitable phenolics) of large molecular weight, that suppress ruminal CH₄ production and bind a moderate amount of protein. The objectives of this study were to evaluate the dose-response effect of replacing alfalfa hay with PA at levels of 0, 25, 50, and 100% on rumen CH₄ production using an in vitro gas production technique and to attempt to elucidate the structure-activity relationship between PA CT and ruminal CH₄ suppression. Leaf tissue was incubated with bovine rumen fluid under anaerobic conditions for 48 h. There was a negative linear relationship between PA inclusion in the diet and in vitro ruminal CH₄ (R² 0.87;P < 0.0001). We also evaluated the subunit composition of PA proanthocyanidins and found them to be uniquely resistant to degradation. Electrospray ionization mass spectrometry showed that PA proanthocyanidin was comprised predominantly of 5-deoxy-flavan-3-ol subunits. The 5-deoxy-subunit decreases reactivity of the interflavan bond contributing to resistance to degradation. We speculate that low reactivity and increased resistance to degradation leads to prolonged life and possibly enhanced activity in the ruminant gastrointestinal tract, which could result in inhibition of methane-producing microbes. Understanding the relationship between structure and activity for proanthocyanidins provides new insights into ruminant gastrointestinal microbial ecology and substantiates the hypothesis that forage CT exhibit structure-specific bioactivities.