Nutritional evaluation of south-west Queensland pastures. 1. The botanical and nutrient content of diets selected by sheep grazing on Mitchell grass and mulga/grassland associations

Abstract

The botanical composition and nutrient content of diets selected by sheep grazing Mitchell grass (Astrebla spp.) and mulga (Acacia aneura)/grassland pastures in south-west Queensland were studied during a four-year period. Forbs, when present in the sward, contributed significantly to the diets on both grassland types. Under drought conditions grasses were the major components of the diets of sheep grazing Mitchell grassland pastures, while under similar conditions on the mulga/grassland pastures at least 35% of the diet was mulga (Acacia aneura); some mulga was present in the diet at all times. In general, the nutrient content of the green leaves of grasses and forbs growing on the mulga/ grassland association was similar, whereas on the Mitchell grass association green forbs had a higher nutrient concentration than did the leaves of green grasses. Both the grasses and forbs on the mulga/ grassland site had lower P and sodium concentrations than those on the Mitchell grassland site. When seasonal conditions were average to good the concentrations of all nutrients in the diets of sheep grazing the Mitchell grass association were judged to be adequate. However, when the pastures dried out the diets contained low concentrations of N (7.0 g/kg DM), P (0.9 g/kg DM), sodium (O.lg/kg DM) and copper (3.8 mg/kg DM). On the mulga/grassland site dietary concentrations of P and sodium were low at all times and under drought conditions the N:S ratio fell to 16.8:1. The concentrations of other nutrients in the diets were high enough to provide the sheep's theoretical requirements at all times.