HOW LAND CONDITION ALTERS PLANT-ANIMAL RELATIONSHIPS IN AUSTRALIA TROPICAL RANGELANDS

Changes in vegetation composition and land condition that occur in res ponse to grazing in natural grassland systems may alter plant and anim al production, but the relationships are poorly understood, This study examined the effect of a change in land condition on herbage and anim al production in two tropical tallgrass regions of northern Australia. The contrasting land condition classes, selected along fence-lines se parating paddocks with different grazing histories, were defined as: S tate I. Dominated by palatable tussock perennial grasses; State II. Le ss palatable tussock perennial grasses, annual grasses and forbs as th e major species. State II produced considerably less herbage than Stat e I. Cattle grazed the two land condition classes to achieve comparabl e utilisation rates over the range 5-60% utilisation in a number of 8 week seasonal grazing periods over 2 years. At both Charters Towers an d Katherine, steer growth was greater in State II than in State I, mos t 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 utilis ation. Owing to the differences in pasture productivity between land c ondition classes, comparable levels of utilisation were achieved by im posing contrasting stocking rates, Steer growth was higher from State II land at low stocking rates while at high stocking rates State I pro duced better liveweight gains. The greater liveweight gain by steers g razing State II at low stocking rates can be explained by better diet quality which was reflected in higher levels of faecal nitrogen. The p roportion of C-3 species (forbs, native legumes, shrubs, eucalypt leav es) in the diet of animals grazing State II vegetation was about twice that of steers in State I, These C-3 species were considerably higher in nitrogen than the C-4 grasses at the end of the wet season. At hig h stocking rates, animals in the State II treatment had less available feed on offer, resulting in lower liveweight gains compared with Stat e I. The likelihood of further deleterious changes to land condition a nd productivity occurring should management attempt to maximise produc tion from State II land are high.