Sodium sulphite effects on recovery and composition of detergent fibre and lignin from forage legumes varying in levels of proanthocyanidins

Alfalfa (Medicago sativa L), red clover (Trifolium pratense L), birdsfoot trefoil (Lotus corniculatus L), sainfoin (Onobrychis viciifolia Scop), crownvetch (Coronilla vana L), cicer milkvetch (Astragalus cicer L), sericea lespedeza (Lespedeza cuneata (Dum-Cours) G Don) and kura clover (Trifolium ambiguum M Bieb) were subjected to sequential detergent fibre analysis to investigate the effects that the addition of sodium sulphite to neutral detergent has on the recovery and composition of fibre and lignin from forage legumes that vary in levels of proanthocyanidin (PA). Soluble, insoluble and neutral detergent insoluble PA (NDIPA) concentrations were highest in sericea, moderate in crownvetch, sainfoin and birdsfoot trefoil and absent in alfalfa, cicer milkvetch, red clover and kura clover. Addition of sodium sulphite reduced levels of neutral detergent fibre (NDF), acid detergent fibre (ADF), acid detergent lignin (ADL), neutral detergent insoluble nitrogen (NDIN) and acid detergent insoluble nitrogen (ADIN) recovered from most forages tested. The addition of sodium sulphite effectively eliminated NDIPA from NDF. The difference between fibre fractions prepared without and with the addition of sodium sulphite during the neutral detergent procedure was related to PA concentration. Neutral detergent fibre difference was positively correlated with soluble PA (r = 0.730, p = 0.0001), insoluble PA (r = 0.905, p = 0.0001) and NDIPA (r = 0.913, p = 0.0001). Acid detergent fibre difference was positively correlated with soluble PA (r = 0.796, p = 0.0001), insoluble PA (r = 0.976, p = 0.0001) and NDIPA (r = 0.974, p = 0.0001). Acid detergent lignin difference was positively correlated with soluble PA (r = 0.846, p = 0.0001), insoluble PA (r = 0.992, p = 0.0001) and NDIPA (r = 0.972, p = 0.0001). Neutral detergent insoluble nitrogen difference was positively correlated with soluble PA (r = 0.475, p = 0.0255), insoluble PA (r = 0.579, p = 0.0047) and NDIPA (r = 0.570, p = 0.0056). Acid detergent insoluble nitrogen difference was positively correlated with soluble PA (r = 0.798, p = 0.0001), insoluble PA (r = 0.969, p = 0.0001) and NDIPA (r = 0.979, p = 0.0001). Sodium sulphite has large effects on fibre values of PA-containing species. Our results suggest that the difference between fibre fractions prepared with and without the addition of sulphite to neutral detergent may be used to determine the effects of PA on protein solubility in detergents.