Influence of tannins on the nutritive value of ruminant feed

Abstract

Tannins are very important secondary metabolites in the plant world. They form an integral part of plant defenses against herbivores, particularly in plants from difficult growing areas. The chemical structure of these polyphenols enables them to bond to many different molecules, mostly proteins. The different types of bonds involved depends on factors relating to the nature of the medium and the structure of the reacting molecules (the presence of proline in the protein enables these linkages). These biochemical characteristics underline the variety and the complexity of the effects of tannins on nutrition. The consequences of tannin ingestion for ruminants include a decrease in their protein degrading ability in the rumen, a perturbation of the microbial activity in both the rumen and caecum, a diminution of intake level and even a toxic effects. Herbivores that are adapted to a diet rich in tannins have developed protective mechanisms, including, in particular, the synthesis of salivary proteins that are rich in proline, which are capable of neutralizing the ingested tannins. The tanning of dietary proteins by natural tannins is a possible technique in ruminant nutrition.