C.K. McDONALD, R.M. JONES and M.W. SILUEY

CSIRO Division of Tropical Crops and Pastures, St Lucia, Queensland, Australia

Summary

Green panic (Panicum maximum var. trichoglume) and rhodes grass (Chloris gayana) were sown on a fertile soil in subcoastal south-east Queensland, Australia, and grazed at 5 stocking rates (1.0 to 2.2 steers/ha), with and without nitrogen fertiliser (100kg N/ha/yr). The occurrence of unsown species within 1m² quadrats on a 25 m × 20 m grid was monitored annually in summer from 1973–1980. Spatial data were analysed by cluster analysis and the number of species per paddock and per quadrat were analysed by regression and analysis of variance.
The number of species per paddock and per quadrat was reduced by the application of nitrogen fertiliser. Increasing stocking rate increased the number of species per quadrat but not per paddock, except at the highest stocking rate.
Factors affecting the composition of unsown species were, in order of importance, soil moisture >> fertiliser > stocking rate.
Soil moisture affected the composition of unsown species in 2 ways. Changes in the availability of winter–spring soil moisture from above average to below average caused a shift in the unsown species present. Drought followed by good winter soil moisture triggered extraordinary pulses of Cirsium vulgare. There was no apparent effect of either the species shift or pulses of unsown species on animal liveweight gain.
The results are examined in terms of biodiversity and selection of species as indicators of overgrazing.