Effect of sheep grazing Astrebla grassland in central western Queensland. 2. Effects of seasonal rainfall

Abstract

The response of Astrebla grassland pasture to summer rainfall was measured at sites in commercially grazed paddocks which were subject to different grazing pressures. The interrelationship between forage use and plant variates was examined by regression analysis. Above-average rainfall resulted in increases in basal area and projected foliage cover of Astrebla spp. under grazing pressures equivalent to 50% forage use of A. lappacea. Increased forage use in the dry period before this rainfall reduced the basal area and projected foliage cover of Astrebla and resulted in lower yields of Astrebla following the rainfall. These lower yields of Astrebla were associated with higher yields of annual grasses and forbs than where forage use had been light. Astrebla yields were highest under light forage use. The frequency of two undesirable species, Amaranthus niitchellii and Tribulus terrestris, was highest under heavy forage use, and was associated with a decline in the number of other annual grass and forb species.