A.J. ASH and J.P. CORFIELD

CSIRO Tropical Agriculture, Townsville, Queensland, Australia

Abstract

Diet selection patterns can have a strong influence on botanical composition in grasslands. This study examined responses in plant selectivity of a preferred (Themeda triandra) and non-preferred (Chrysopogon fallax) perennial grass to a change in pasture condition in a monsoon tallgrass community in northern Australia. Two contrasting pasture condition classes were defined: State I — dominated by palatable tussock perennial grasses; State II — dominated by less palatable tussock perennial grasses, annual grasses and forbs.
Themeda was selected actively by cattle in all seasons, except in the late dry. Pasture condition interacted with season of grazing to alter significantly the selectivity for Themeda. In the early and late wet seasons, selection for Themeda was significantly greater in State II than in State I. As the proportion of Themeda decreases (State I to State II), additional selection pressure is placed on the remaining plants, which is likely to hasten their loss from the system. Increasing the level of utilisation, in an effort to distribute the grazing pressure more evenly, had little effect on reducing the relative preference for Themeda. In State I, Chrysopogon was relatively avoided in all seasons, especially in the late wet and early dry. However, in State II, Chrysopogon was selected actively in all seasons except the late wet.
Grazing management strategies designed to increase the proportion of a palatable species in a pasture must take account of diet selection patterns. Simply reducing stocking rate to a conservative level may not allow rapid recovery of Themeda populations because of continued preference for Themeda. Spelling of pasture over the wet season may offer the best means of manipulating pasture composition in monsoon grassland communities.