Tannins (hydrolysable and
condensed tannin) are polyphenolic
polymers of relatively high molecular weight with the capacity to form complexes mainly with
proteins due to the presence of a large number of phenolic
hydroxyl groups. They are widely distributed in nutritionally important forage trees, shrubs and legumes, cereals and grains, which are considered as anti-nutritional compounds due to their adverse effects on intake and animal performance. However,
tannins have been recognised to modulate rumen fermentation favourably such as reducing protein degradation in the rumen, prevention of bloat, inhibition of methanogenesis and increasing
conjugated linoleic acid concentrations in ruminant-derived foods. The inclusion of
tannins in diets has been shown to improve
body weight and wool growth, milk yields and reproductive performance. However, the beneficial effects on rumen modulation and animal performance have not been consistently observed. This review discusses the effects of
tannins on
nitrogen metabolism in the rumen and intestine, and 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 of responses of
tannins among different studies are attributed to the different chemical structures (degree of polymerisation,
procyanidins to propdelphinidins, stereochemistry and C-C bonding) and 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.