Digestibility of some tropical browse species varying in tannin content

The nylon bag dry matter digestibility (NBDMD) of one grass and 12 tropical shrubs (10 legumes and two non-legumes) varying in tannin content was measured in the rumens of five Droughtmaster steers. Five terminal and fully expanded leaves of each shrub were harvested by hand plucking, chopped fresh to about 3 mm lengths and incubated (either as fresh or after oven-drying at 65 degrees C for 24 h) for 48 h. Chemical contents of freeze-dried samples varied widely among species: 1.59-3.84% for N, 25.93-53.21% for NDF, 19.17-39.44% for ADF and 1.97-19.36% for ADL. Extractable condensed tannin levels ranged from 0.3 to 11.6% for vanillin/HCl and from 0 to 9.3% for butanol/HCl extraction. Bound tannins also varied, the protein bound component of both Gliricidia sepium and Bauhinia rufescens exceeding 2% of the dry matter. B. rufescens also had a high fibre-bound tannin level of 0.9%. Oven drying consistently increased the NDF values and also increased the ADF and ADL levels in all species except Calliandra calothyrsus where there was a slight depression. However, the effect of oven drying on digestibility with this species was dramatic - 78% for fresh versus 41% for dried. For most of the other species NBDMD was similar for fresh and dried samples. NBDMD differed among species (p<0.001) from 18 to 78% for dried material and from 20 to 78% for fresh material. It was generally low in high tannin species and vice versa except for Callinndra calothyrsus which had high NBDMD for fresh material and high tannin, and Acacia currasavica which had low NBDMD but only average tannin levels. Only 5 of the 10 leguminous browse species evaluated were considered to be useful in sown pastures on the basis of their digestibility these were Leucaena leucocephala, Gliricidia sepium, Albizia lebbek, Albizia richardiana and Calliandra calothyrsus. The non-leguminous, non-tanniniferous Combretum erythrophloeum had the highest NBDMD of all the browse species tested. Significant correlations existed between NBDMD and NDF, ADF, ADL and extractable tannins but not with the bound tannins. (C) 1998 Elsevier Science B.V.