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.

Climate Change in Queensland's Grazing Lands: Ii. An Assessment of the Impact on Animal Production From Native Pastures.

“…Exceptions to this generality occur where the animals are able to select high quality diets from different plant components, plant species within the sward, edible shrubs, or landscape units within a paddock. C 3 species (grasses and forbs) can, at times, make up an important high quality component of an animals diet (Beale 1975, McMeniman et al 1986a. Grazing history and pasture management (burning, tree regrowth control) can also have major impacts on botanical composition and hence diet quality (e.g.…”

“…Grazing history and pasture management (burning, tree regrowth control) can also have major impacts on botanical composition and hence diet quality (e.g. McMeniman et al 1986a, b, Orr 1986, Ash et al 1995. Because animal nutrition is likely to be limited by the availability of plant components of adequate nutrient levels during dry or winter periods, graziers may have to adopt relatively low stocking rates to achieve reproductive rates high enough to maintain self-replacing herds and flocks.…”

Sheep productivity in an Astrebla grassland of south-west Queensland.

“…If heavy rainfall occurs in the following summer, there is a further shift towards perennial grass dominance at the expense of the annual grass and for3 component (Orr 1980(Orr , 1981. Sheep grazing Astrebla grassland preferentially graze forb species when they are available in the pasture (Lorimer 1978, McMeniman et al 1986a). Sheep liveweight is correlated positively with the forb content and negatively with the Astrebla spp.…”

Effect of herbage in Astrebla spp. pastures in northwest Queensland on lamb survival

“…The results of these experiments indicate that a reduction in the availability of forbs in the pasture grazed by pregnant ewes is likely to improve lambing performance when the forb content in Mitchell grass pastures is between 7 and 49 % of total pasture biomass. Forb species are more nutritious than the grasses that grow on the Mitchell grass downs (Lorimer 1976;McMeniman et al 1986), and in pastures of low forb content (2-9%) Orr et al (1988) suggested that increased reproductive performance may be caused by an improvement in diet quality associated with a higher yield of forbs. A lower C 3 content in the diet of ewes grazing paddock F"2 in Expt 3 than in other paddocks shows that they consumed a diet low in palatable C 3 forbs.…”

“…Forbs (herbaceous plants other than grasses) generally constitute a minor proportion of total The high mortality of lambs between birth and lamb pasture biomass; however, the presence of lesser marking (36%) (Moule 1954) in north-west Queens-pasture constituents in the diet of sheep in the subland has been attributed to heat stress (Hopkins et al tropics (Weston & Moir 1969;Lorimer 1978;1980;Stephenson et al 1980) and poor nutrition McMeniman et al 1986), semi-arid environments (Stephenson et al 1981). The average ovulation rate (Leigh & Mulham 1964;Graetz & Wilson 1980; of ewes is 110% (Entwistle 1972), the pregnancy rate Squires 1980) and higher rainfall rangelands (Leigh & of ewes teased using testosterone-treated wethers is Holgate 1978) is significant in terms of productivity.…”