There is little information on the grazing capacity of mined lands rehabilitated to pastures in tropical and sub-tropical regions. In the sub-tropical region of central Queensland, no studies have been conducted to determine the grazing capacity of pastures rehabilitated following open-cut coal mining, despite the success in establishing permanent pastures since the early 1970's. To provide this information, the net primary productivity (NPP) of rehabilitated pastures was determined, and results were related to grazing trials established at rehabilitated sites on three mines in the Bowen Basin.
Seventeen sites at three open-cut coal mines were used to determine the NPP and the rainfall use efficiency (RUE) of rehabilitated pastures across the Bowen Basin over the 2000/2001 growing season. This study indicated that rehabilitated pastures have comparable NPP and RUE to pastures on unmined lands, refuting the hypothesis that rehabilitated pastures in tropical and sub-tropical regions were inferior to similar pastures on unmined lands (Chapter 4). Sites at Norwich Park mine showed the highest mean peak DM yield (7550 kg) followed by Goonyella Riverside mine (4380 kg) and Blackwater mine (4130 kg). Mean RUE values for the sites at each mine were the highest for Norwich Park mine (14 kg/ha/mm) followed by Blackwater mine (13 kg/ha/mm) and Goonyella Riverside mine (6 kg/ha/mm). RUE values for sites at Goonyella Riverside mine were lower than RUE values for sites at Blackwater and Norwich Park mines as a result of a combination of factors that influence water retention and infiltration.
An empirical model was developed that enabled prediction of RUE (r2=81%), and thereby DM production (r2=77%), of rehabilitated pastures in central Queensland. The model included slope, exchangeable Mg, exchangeable sodium percentage (ESP), clay fraction and cation exchange capacity (CEC). The RUE model was developed from a limited number of sites, and additional data is required to improve the capacity of the model to predict RUE and DM yield at other rehabilitated mine sites across the Bowen Basin. Validation of the model using independent data source is paramount.
One intensive and two extensive grazing experiments conducted at the three mines indicated the excellent potential of rehabilitated pastures as a grazing resource, provided that appropriate management was implemented (Chapters 5 and 6). The intensive grazing experiment at Goonyella Riverside mine examined the impact of eight stocking rate treatments (1.0, 1.4, 1.9, 2.6, 3.7, 5.2, 7.2 and 10.1 ha/head) derived from rotational grazing of small relatively uniform paddocks. The preliminary results showed that a stocking rate of 3.7 ha/head was sustainable. The extensive trials comprised large paddocks continuously stocked at a range of stocking rates. These were monitored only over the first year of a long-term study and a longer time frame is required to observe treatment effects. The stocking rate information that was collected from these grazing studies agrees with stocking rates recommended for the region, although there is a need to confirm these results over an extended timeframe. Mean daily liveweight gain of steers at Blackwater mine were 0.65, 0.72 and 0.65 kg/head/day for the HSR, MSR and LSR treatment paddocks, respectively. Steers at Norwich Park mine extensive trial achieved a mean daily liveweight gain of 0.38, 0.39 and 0.49 kg/head/day for HSR, MSR and LSR treatment paddocks, respectively.
Safe stocking rates for the three trial sites, calculated using RUE and long-term seasonal rainfall, indicated the inherent grazing capacity of these pastures (Chapter 7). Sites at Blackwater and Norwich Park mines had safe stocking rates of 2.2 ha/head and 2.7 ha/head respectively, close to the recommended stocking rate for pastures on unmined lands in the region, which is 2-3 ha/head. The safe stocking rate for the trial site at Goonyella Riverside mine was lower (5.9 ha/head) as a result of its inherently low RUE. The stocking rate treatments chosen for the long-term extensive trial at Blackwater mine, were found to be appropriate for long term study (2.4, 3.7 and 7.0 ha/head for the high stocking rate (HSR), medium stocking rate (MSR) and low stocking rate (LSR) treatments, respectively). However, the HSR and MSR treatments for the long-term extensive trial at Norwich Park mine (3.4 and 4.6 ha/head for HSR and MSR treatments, respectively) were unlikely to impose sufficient pressure on the pasture system to create measurable treatment effects.
Grazing trials need to be continued over a minimum 4-5 year time period to validate these preliminary conclusions. NPP data from an expanded range of sites are required to improve the models predicting RUE and DM yield.