Condensed tannins and the nutritive value of herbage

Abstract

Many plant species contain condensed tannins (CT), but lotus is one of the few herbages of agricultural importance to contain CT. Lotus has a high nutritive value (NV) despite a moderate protein content and nitrogen digestibility, and this can be explained by the activity of CT during digestion. CT binds plant proteins in the rumen, reducing their solubility and degradation by rumen bacteria. Condensed tannins increase the passage of plant protein to the intestine, and have increased the availability and absorption of essential amino acids by 60% compared with equivalent CT-free forage. Condensed tannins prevent bloat. Dietary concentrations as low as 0.17% CT in the dry matter (DM) can affect protein solubility in the rumen, but concentrations up to about 2-3% of dietary DM are probably optimal for maximising NV. Values exceeding 5.5% of DM inhibit microbial activity excessively and depress voluntary intakes. Ruminant production of milk, meat and wool could be increased by l0-15 % if grazed pasture contained 2-3%CT. These levels would be achieved if white clover could be engineered to contain 7-8% CT in its foliage. Keywords condensed tannins, lotus, nutritive value, herbage, ruminant digestion