Dietary Tannins on Microbial Ecology of the Gastrointestinal Tract in Ruminants

This review discusses the effects of tannins on nitrogen metabolism in the rumen and intestine, microbial populations (bacteria, protozoa, fungi and archaea), metabolism of tannins, microbial tolerance mechanisms to tannins, inhibition of methanogenesis, ruminal biohydrogenation processes and performance of animals. The discrepancies in responses of tannins among different studies are attributed to the different chemical structures (degree of polymerization, procyanidins to propdelphinidins, stereochemistry and C–C bonding), different concentrations of tannins, and type of diets. An establishment of structure-activity relationship would be required to explain differences among studies and obtain consistent beneficial tannin effects. This paper reviews progress with plant tannins occurring in both temperate and tropical forages for fulfilling the objective of mode of action of tannins, rumen microbial activity and rumen metabolisms.