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The results of a grazing experiment in which Townsville stylo was oversown into native pasture in sub-coastal northern Queensland are reported. The region receives 650 mm average annual rainfall compared with >850 mm at other experimental sites where Townsville stylo has been tested under grazing. The experiment included three treatments in a factorial combination: timber clearing or not, application of 125 kg ha-1 superphosphate or not and stocking rates of 0.4 and 0.2 beasts ha-1. The yield of Townsville stylo depended on treatments and annual rainfall but was generally low in this environment. Over the ten year period of the experiment the pastures remained dominated by Heteropogon contortus and Bothriochloa bladhii, and there was no invasion of annual grasses. Clearing of the timber increased the pasture yield by 77%. There was no regrowth of trees after clearing. Within each year cattle gained weight in the wet season, when pasture quality was high, and lost weight in the dry season, when pasture quality declined. Mean liveweight gain over all treatments was correlated (r = 0.97) with the length of the growing season. A significant response in liveweight gain to fertilizer occurred only in years of average or above rainfall when there was also a response in Townsville stylo yield. The effect was due to increased gains during the wet season; there were no differences in liveweight gain during the dry season, when Townsville stylo became decomposed. Significantly greater liveweight losses occurred on the heavily stocked treatments during the dry season of drought years. The yield of Townsville stylo in the early years of the experiment was significantly higher on the cleared treatments and there was a corresponding response in liveweight gains in the wet season during this period. Liveweight losses in the dry season were also significantly higher on the timbered treatments in years of drought. The powerful influence of climatic variation on the treatments places reservations on their practical application. The increased pasture yield from tree clearing is likely to be a benefit to cattle only in years of drought. The responses to superphosphate fertilizer only in years of high rainfall suggests that its use is unlikely to be profitable on Townsville stylo based pastures in this environment.

Summary. An expert system has been developed, using the results from the National Reactive Phosphate Rock Project, to determine whether reactive phosphate rock is likely to be an effective substitute for water-soluble superphosphate fertiliser for a given pasture environment. The evaluation is made from site information [annual rainfall, pasture composition and the likelihood of phosphorus (P) leaching], and soil information (pH, Colwell P, soil colour and field texture). The expert system can determine the effectiveness of both highly reactive and moderately reactive phosphate rocks. Observed substitution values of triple superphosphate for the highly reactive North Carolina phosphate rock (ratio of the respective P levels required to produce 50% of the maximum observed yield response to triple superphosphate) were closely related to values predicted by the expert system (r = 0.92); the relationship between observed and predicted substitution values of single superphosphate for the moderately reactive Hemrawein phosphate rock was also close (r= 0.86). The expert system gives one of 4 different recommendations for reactive phosphate rock based on the magnitude of the predicted substitution values. These are ‘immediately effective’, ‘effective in the medium term’, ‘marginally effective’, and ‘not effective’. The system was validated using the results from independent field experiments that provided measures of the effectiveness of reactive phosphate rock at different pasture sites.