The effect of condensed tannins on the nutrition and health of ruminants fed fresh temperate forages: a review

The effect of condensed tannins (CT) on the nutritive value of temperate forages and on the health of grazing ruminant animals is reviewed. The CT bind with proteins and other entities mainly by hydrophobic and hydrogen bonding in a pH-reversible manner, which is influenced by the structure and molecular weights of both the CT and the proteins. These reactions can be used to reduce the degradation of forage proteins in the rumen, without reducing the amount of microbial protein synthesized. CT in Birdsfoot trefoil (Lotus corniculatus) and sulla (Hedysarum coronarium) can be used to increase the absorption of essential amino acids (EAA) from the small intestine, but CT in Big trefoil (Lotus pedunculatus) and sainfoin (Onobrycbis coronarium) produce no net increase; differences in CT structure and reactivity probably explain these differences. Low concentrations of CT (20-45 g CT/kg DM) reduce rumen forage protein degradation due to reversible binding to these proteins and to reducing the populations of proteolytic rumen bacteria. CT in several forage plants (e.g. L. corniculatus and sulla) have been shown to offer advantages for ruminants, and have resulted in increased in milk production, wool growth, ovulation rate, and lambing percentage, as well as reducing bloat risk and reducing internal parasite burdens. This is probably related to action of the CT in increasing EAA absorption from the small intestine; in the case of internal parasites their inactivation by CT is also involved. However, high forage CT concentrations (>55 g CT/kg DM) generally reduce voluntary feed intake and digestibilities, and depress rates of body and wool growth in grazing ruminants. It was concluded that moderate concentrations of CT can be used to increase the efficiency of protein digestion and to improve animal health under grazing, producing more sustainable grazing systems. The effects produced are not the same for all CT, but rather depend upon the concentration and structure of the CT. (C) 2003 Published by Elsevier Science B.V.