DETERMINATION OF EXTRACTABLE AND BOUND CONDENSED TANNIN CONCENTRATIONS IN FORAGE PLANTS, PROTEIN-CONCENTRATE MEALS AND CEREAL-GRAINS

A procedure was developed for extraction of 'free' condensed tannins (CT) using a mixture of acetone/water/diethyl ether (4.7:2.0:3.3). followed by extraction of protein-bound and fibre-bound CT using boiling sodium dodecyl sulphate containing 2-mercaptoethanol (SDS). CT concentrations in all three fractions were determined by a modified butanol HCl procedure. Separate standard curves using purified CT in water or SDS solution were utilised for analysis of extractable CT (water standards) and protein-bound and fibre-bound CT (SDS standards). The method accurately predicted the concentration of CT added to forage extracts. CT extractable in acetone/water/diethyl ether comprised, on average, 68% of total CT in a range of freeze dried forage legume samples, with most of the remainder being bound to protein. When total CT concentration was low (0.6-3.0% DM), a lower proportion was extractable (33-35 %). In protein concentrate meals containing CT, the extractable, protein-bound and fibre-bound components comprised 15, 60 and 25% respectively of total CT. Total CT concentration in the forages Lotus corniculatus and Coronilla varia was considered appropriate for ruminant nutrition (2.1 and 3.0% DM), whilst CT concentration in the forage of Dorycnium spp (13-19% DM) was more suitable for soil conservation purposes. The substantial CT concentration in cottonseed meal (1.6 % DM) may be involved in the high resistance of proteins in this product to ruminal degradation. CT concentration was indistinguishable from zero in perennial ryegrass forage, in barley and triticale grains and in soya bean meal (0.1 % DM).