Drying method and origin of standard affect condensed tannin (CT) concentrations in perennial herbaceous legumes using simplified butanol-HCl CT analysis

BACKGROUND: Several factors affect condensed tannin (CT) levels in plants and accuracy of the butanol-HCl assay for total CT. Six native, perennial, herbaceous legumes from Texas were harvested at three stages of growth over a growing season; young vegetation, initial flowering, and late season. The samples were subjected to oven-drying and freeze-drying and analyzed for extractable (ECT), protein-bound (PBCT), and fiber-bound (FBCT) CT using a butanol-HCl procedure, comparing several types of purified CT as standards. RESULTS: There was a wide range of CT concentration in the legumes, but little change in ECT and total (TCT) tannin concentrations over the growing season. Oven-drying decreased (P < 0.05) ECT and increased (P < 0.05) PBCT and FBCT concentrations compared with freeze-drying, but both methods ranked the forages similarly throughout the growing season relative to TCT. Quebracho tannin as a standard increased CT values for the forages compared with purified legume CT standards, but all standards ranked the forages similarly relative to tannin concentration. Modifications to the procedure increased speed and ease of using the CT analysis method. CONCLUSION: Quantitative CT values have limited value except to allow a relative ranking of forages. Selection of appropriate standards is important in providing realistic CT values. Improvements in the total (extractable + bound) CT analysis method will allow wider use of this procedure. Although ECT and TCT values changed little in native legumes over a growing season, biological activity and potential nutritional/medicinal benefits of CT in these plants needs further evaluation. (c) 2008 Society of Chemical Industry.