Chemical composition, biological effects of tannin and in vitro nutritive value of selected browse species grown in the West Australian Mediterranean environment

Acacia saligna, Chamaecytisus palmensis, Atriplex amnicola and Atriplex nummularia are major cultivated browse forage species in Western Australia (WA). Rhagodia eremaea, a browse species indigenous to the southern rangelands of WA is currently being investigated as a potential browse species for cultivation as a feed resource for ruminants. The chemical composition and nutritive value of these browse species, when grown in the Mediterranean environment of WA were examined and compared with that of A. sativa hay, which is commonly used in ruminant feeding systems in Australia. All were analyzed for nutrient composition, total phenolics (TP), and total tannin (TT) and mineral composition. In vitro gas production was measured until 24 h, with and without the addition of polyethylene glycol (PEG). The organic matter digestibility (OMD), and metabolizable energy content (ME) were estimated. Browses all contained higher levels of CP (96.7–216.7 g/kg DM vs 52.5 g/kg DM) but lower levels of neutral detergent fiber (233.0–487.8 g/kg DM vs 611.6 g/kg DM) than A. sativa. The browses were rich in most minerals including Ca, Mg and Zn, which were deficient in A. sativa. The content of lignin(sa) was highest in A. saligna (123.2 g/kg DM). Although, C. palmensis had the highest level of TP (44.3 g/kg DM), A. saligna had the highest level of TT (28.9 g/kg DM). While phenolics in A. sativa and halophytes species (A. amnicola, A. nummularia, R. eremaea) were very low, the latter was extremely high in ash (156.9–178.8 g/kg DM). Significant improvement of in vitro gas parameters with the addition of PEG observed only in A. saligna. The ash, lignin(sa), TP and TT contents in the leguminous species (A. saligna, C. palmensis) were negatively correlated with in vitro nutritive characteristics (r > −0.78). C. palmensis was higher in OMD (667.5 g/kg DM) and ME (10.0 MJ/kg DM) compared to the other browse species. The nutritive value (OMD, ME) of A. saligna was limited due to higher TP and lignin(sa) contents while that of the halophytes was limited by higher ash and, presumably, high soluble N contents. All of the browse species were rich in CP and minerals and therefore useful supplements for low CP forage diets. With the exception of C. palmensis, the other browses were poor in OMD and ME and therefore would not be suitable as a sole diet for sheep.