OCCURRENCE, NUTRITIONAL SIGNIFICANCE AND EFFECT ON ANIMAL PRODUCTIVITY OF TANNINS IN TREE LEAVES

The presence of tannins in a large number of nutritionally important tree leaves hampers their utilization as ruminant feed. Generally, both the condensed and hydrolysable tannins are present in tree leaves, but some tree leaves may contain predominantly condensed tannins whereas others contain hydrolysable tannins. An inverse relationship has been found between the protein-precipitating capacity of tannins in tree leaves and their palatability, voluntary intake and digestibilities of crude protein and dry matter in grazing as well as browsing animals. Dietary tannins adversely affect rumen metabolism by bacteriostatic and bactericidal activities, and by inactivating several enzymes (e.g. carboxymethyl cellulase, proteases, glutamate dehydrogenase). The availability of sulphur and iron becomes limited to the animals consuming tannin-rich tree leaves. The prolonged consumption of tannin-rich leaves induces toxicity. Sheep ingesting 0.9 g hydrolysable tannins kg-1 body weight might show signs of toxicity in 15 days. Although animals like mule deer, rats and mice have been shown to secrete proline-rich proteins in saliva and these constitute the first line of defense against ingested tannin, the observed deleterious effects suggest the inadequacy of defence against a high quantity of dietary tannins. A tannin content of ∼ 5% in browse may not only act as a feeding deterrent, but also influence the digestibilities. Hence when such leaves are offered as a sole feed they may lower the animal's productivity in terms of body weight gain and wool yield. Treatment with polyethylene glycol-4000 seems viable in reducing the deleterious effects of tannins in top feed, however its economical field application remains to be elucidated. © 1990.