Tannins are polyphenolic commands and the secondary metabolites of higher plants. Tannins are generally classified as condensed tannins, hydrolysed tannins and phlorotannins. Condensed tannins consist of flavan-3-ol subunit linked together to form oligomers and polymers, whereas hydrolysed tannins are easters of gallic or ellagic acid linked to polyol core.
Phlorotannins are structurally less complexed and occur only in marine brown algae. Because secondary metabolites serve as a part of plant chemical defence system against invasion by pathogens and attack by insects, tannins have shown antimicrobial, anti-parasitic, antioxidant, anti-inflammatory and anti-virus properties. Recently it was reported that condensed tannins significantly reduced methane emission, but hydrolysed tannins failed to affect methane reduction. However, both condensed tannins and hydrolysed tannins reduced Nitrous Oxide emission.
Feedlot and pasture frothy bloat is a serious and often fatal digestive disorder that develops in cattle or sheep when gas produced during fermentation of feeds is trapped within the rumen in the stable form and prevent eructation. Moderate levels of condensed tannins (<50 g/kg DM) reduce protein degradation in the rumen without depressing rumen fibre digestion or DM feed intake. It is generally believed that condensed tannins may bind protein in the rumen and reduce foam stability by avoiding microbial degradation and be absorbed in the small intestine.
Tannins exert their anti-parasitic effect by decreasing the viability of larvae, thus interfering with egg hatching, and improving the immunity (Table 1). Some tree bark is rich in plant second metabolites, especially condensed tannins.
Recently it was demonstrated that the tree bark-stripping behaviour is primarily for ruminants to acquire condensed tannins to repel gastrointestinal parasites.
Table 1 Effect of grazing condensed tannins forages and control forages on parasite state.