Polyphenolics and tannins effect on in vitro digestibility of selected Acacia species leaves

Browse tree leaves from six species of Acacia being: A. angustissima, A. drepanolobium, A. nilotica, A. polyacantha, A. tortilis and A. senegal were screened to quantify levels of extractable total phenolics (TEP), extractable tannin (TET) and condensed tannin (CT). The CT in the leaf samples were assayed for soluble, protein-bond and fibre-bound CT using a modified butanol/HCl technique through improved extraction of tannin in leaf samples with aqueous sodium deodocyl sulphate (SDS)-β-mercaptoethanol solution. Effect of tannins on in vitro organic matter digestibility (OMD) was assessed by polyethylene glycol (PEG) tannin bioassay. Forages had variable crude protein (CP) ranging from 176 g/kg DM in A. nilotica to 229 g/kg DM in A. angustissima. A. nilotica had the lowest (P < 0.05) neutral detergent fibre (NDF), acid detergent fibre (ADF) and acid detergent lignin (ADL) of 222, 134 and 55 g/kg DM, respectively, compared to A. polyacantha (505, 393 and 196 g/kg DM, respectively). The TEP varied among species from 99 in A. drepanolobium to 281 mg/g DM in A. nilotica. TET also varied from 84 (A. drepanolobium) to 256 mg/g DM in A. nilotica. Total CT varied between Acacia spp., and ranged from 52.8 in A. nilotica to 98.3 mg/g DM in A. polyacantha. Most of CT in Acacia spp. was bound to protein (22.2-50.5 mg/g DM). Soluble and fibre-bound CT fractions varied (P < 0.05) among the species from 14.5 to 22.9 mg/g DM and 13.0 to 28.6 mg/g DM, respectively. Addition of PEG in vitro increased gas production, OMD and metabolisable energy (ME) content with the highest response (P < 0.05) in A. angustissima compared to other Acacia spp. High levels of phenolics and tannins in Acacia spp. samples could limit utilisation of Acacias by ruminants through impaired feed digestibility and nutrient utilisation.