Content of phenolics and tannins in leaves and pods of some Acacia and Dichrostachys species and effects on in vitro digestibility

Four browse legume foliages (leaves and pod fruits) from three species of Acacia (A. polyacantha, A. tortilis, A. nilotica) and Dichrostachys sp. native to Tanzania were evaluated for nutritive potential to establish levels and effect of normal phenolics and tannins antinutritive factors (ANFs) on in vitro feed digestibility (IVD). Total extractable phenotics (TP), tannins (TT) were estimated by Folin-Ciocalteu assay, and condensed tannins (CT) by butanol/HCl assay. In leaves, total proanthocyanidins were assayed into constituent anthocyanidins' flavonoids by high performance liquid chromatography (HPLC). Organic matter degradability (OMD) was estimated in vitro by gas production technique. Adverse effects of tannin's ANF on IVD were assessed by polyethylene glycol (PEG) tannin bioassay. Crude protein (CP) varied (P<0.05) between fodder species and foliages. Leaves had (P<0.05) higher CP (141-194 g/kg DM) in Dichrostachys sp. and A polyacantha, respectively, compared to pods (133-142 g/kg DM) in A. tortilis and Dichrostachys sp., respectively. Neutral detergent fibre (NDF), acid detergent fibre (ADF) and acid detergent lignin (ADL) varied (P<0.05) between fodder species and foliages. A. nilotica had lowest (P<0.05) NDF, ADF and ADL (182, 68 and 44 g/kg DM, respectively). A. polyacantha and Dichrostachys sp. had highest (P<0.05) NDF and ADF (416 and 146 g/kg DM, respectively). Pods had higher (P<0.05) fibre values than leaves. Leaves had higher (P<0.05) TP, TT, and CT (162, 138, and 55 mg/g DM), than pods (112, 105 and 35 mg/g DM), respectively. Proanthocyanidin flavonoids (mg/g DM) varied (P<0.05) from 0.062 (A. nilotica) to 5.288 (A. tortilis), 0.188 (A. tortilis) to 4.179 (A. polyacantha), and 0.009 (A. nilotica) to 4.392 (Dichrostachys sp.) for delphinidins, cyanidins and pelargonidin, respectively. The browse foliages had relatively high tannin content greater than 5% DM, a beneficial level in animal feeding and nutrition, and thus could impair feed digestibility. OMD varied significantly (P<0.05) both between fodder species and foliage components. Pods had higher (P<0.05) gas production (GP) and OMD than leaves. Addition of PEG improved (P<0.05) gas production, IVD and metabolizable energy (ME) in leaves and pods by binding tannins. Leaves had higher response on gas production and IVD due to PEG treatment compared to pods except A. nilotica. Improved gas production and digestibility due to PEG treatment indicate nutritive potential in browse fodder previously depressed by tannin ANFs. Therefore, phenolics and tannins ANFs could limit utilization of browse fodder nutritive potential as supplements to ruminants consuming low quality roughages. Utilization of browse legume fodder could be optimized through reduction of tannin antinutritive activity. Under farmers' conditions, optimal utilization of browse could be achieved through feeding a mixture of tanniferous browse with other feeds especially high in nitrogen to dilute tannin antinutritive activity.