How land condition alters plant-animal relationships in Australia's tropical rangelands

Abstract

Changes in vegetation composition and land condition that occur in response to grazing in natural grassland systems may alter plant and animal production, but the relationships are poorly understood. This study examined the effect of a change in land condition on herbage and animal production in two tropical tallgrass regions of northern Australia. The contrasting land condition classes, selected along fence-lines separating paddocks with different grazing histories, were defined as: State I. Dominated by palatable tussock perennial grasses; State II. Less palatable tussock perennial grasses, annual grasses and forbs as the major species. State II produced considerably less herbage than State I. Cattle grazed the two land condition classes to achieve comparable utilisation rates over the range 5-60% utilisation in a number of 8 week seasonal grazing periods over 2 years. At both Charters Towers and Katherine, steer growth was greater in State II than in State I, most rapid in the early wet season and least in the late dry season, and greatest at low utilisation rates. Differences in liveweight gain due to land condition class were greatest at the lightest levels of utilisation. Owing to the differences in pasture productivity between land condition classes, comparable levels of utilisation were achieved by imposing contrasting stocking rates. Steer growth was higher from State II land at low stocking rates while at high stocking rates State I produced better liveweight gains. The greater liveweight gain by steers grazing State II at low stocking rates can be explained by better diet quality which was reflected in higher levels of faecal nitrogen. The proportion of C3 species (forbs, native legumes, shrubs, eucalypt leaves) in the diet of animals grazing State II vegetation was about twice that of steers in State I. These C3 species were considerably higher in nitrogen than the C4 grasses at the end of the wet season. At high stocking rates, animals in the State II treatment had less available feed on offer, resulting in lower liveweight gains compared with State I. The likelihood of further deleterious changes to land condition and productivity occurring should management attempt to maximise production from State II land are high. © 1995.