Forage tannins in ruminant nutrition

Chemically distinct, hydrolysable and more stable condensed type of tannins have been identified in forages. The values depend on the analytical method used. Factors affecting tannin content of foliage were: climatic factors (higher in tropical species and under drier conditions), soil fertility, species (shrubs and tree species having higher levels than pasture types), stage of growth (less mature having higher levels) and plant part of significance was the accessibility to browsing and the act itself resulting in higher tannin levels, recognized as a defense mechanism mediated from affected plants to others through ethylene produced due to browsing to increase tannin levels in unaffected plants to discourage browsing. Younger parts and those accessible for browsing having higher levels are other defense mechanisms. Proline in saliva of deer binds with tannin in shrubs and tree species offering a counter mechanism by browsers while the saliva of grazers (cattle and sheep) is proline-free. Depending on the level of tannins in a diet, the effect may vary from beneficial (2-4%) through antinutritional (5-9%) to toxic and lethal (>9%). Effects are mainly due to complexing with enzymes/protein thereby reducing rumen NH₃ microbial activity, nitrogen and dietary dry matter utilization, resulting in decreased intake and animal performance. Polyethylene glycol (PEG) in feed preferentially binding condensed tannins improved the utilization of protein more than that of the dry matter with the effect being greater at higher tannin levels. Split compared to single dose PEG seemed more effective. Limited evidence indicated that PEG promotes selection and intake of high tannin forages. Attempts to imp rove silage quality using tannins have also been reported.