Open Cut » Environment
Grazing cattle defoliate a pasture through direct consumption, and through trampling as they move around. If grazing pressure is low, then the pasture is able to recover with no loss in long-term yields or ground cover. Light grazing may even stimulate the pasture and improve productivity compared to an ungrazed pasture. However, above a certain 'critical level' of utilisation, the pasture is unable to maintain regrowth. Cover declines, plants contract in size (above and below-ground) and become less vigorous and the soil is exposed to erosion and loss of nutrients. Over a prolonged period, changes in species composition can also occur with weeds and less palatable grasses becoming more prevalent. The 'critical level' is the maximum long-term sustainable carrying capacity, which is determined by the broad quality of the pasture environment.
STAGE 1 PROJECT SUMMARY
The two-year project officially commenced in February 2000. The program consisted of three grazing trials established at Goonyella Riverside, Norwich Park and Blackwater mines on sites that varied in soil type and age since establishment (3 - >25 years). At Goonyella Riverside, a short-term but intensive grazing experiment was designed to rapidly create a range of carefully controlled degradation states. Cattle were rotated through small paddocks (up to 0.8 ha in size) in two periods of four weeks to simulate a total of eight different grazing pressures. Available forage, cover and species composition were monitored throughout the trial period.
The trials at Norwich Park and Blackwater mines were set up under continuous grazing regimes testing three grazing pressures in paddocks up to 35ha in size. Both trials were designed to validate and expand on the conclusions drawn from the intensive short-term grazing experiment at Goonyella Riverside mine, and to provide demonstration of practical management outcomes. Again, available forage, cover and species composition were monitored before and following the introduction of cattle in February 2000 (Blackwater) and May 2000 (Norwich Park).
Under the two-year time horizon, assessment of the three trials can provide preliminary indicators of grazing capacity. A study was undertaken to develop a model to predict long-term sustainable stocking rates. The model predicted grass production using the concept of rainfall-use efficiency, or the ability of a pasture to produce forage per unit of rainfall. With additional long-term rainfall records and some assumptions, 'safe' stocking rates can be calculated. Predictions for each site were compared with likely suitable grazing pressures observed in the trials. The study involved estimation of peak seasonal pasture growth in ungrazed plots on all three sites, with results linked to measured site and soil quality parameters.
Stage 1 demonstrated that pasture-based rehabilitation on open-cut coal mines in central Queensland has the capacity to support cattle grazing. Estimated sustainable stocking rates on pasture rehabilitation sites at Blackwater, Norwich Park and Goonyella Riverside mines were 2.7, 2.2 and 5.9 ha/head, respectively. Estimates were based on a model of pasture productivity which predicted the amount of feed available for cattle consumption, yet left sufficient cover for erosion control. The predicted sustainable stocking rates were in agreement with preliminary recommended rates from the three grazing trails, and were comparable with stocking rates reported for improved pastures on unmined land in the region.
STAGE 2 – PROJECT SUMMARY
This project aimed to build confidence in determining whether grazing was likely to be a sustainable land use option in the rehabilitated landscapes of the Bowen Basin following open-cut coal mining. The project was designed to capitalise on the substantial ACARP-funding and site in-kind and cash support assembled for the preliminary (Stage 1) study. As such, this research builds on the success of ACARP Projects C8038 and C9038 (Stage 1) by continuing to utilise the existing grazing trials and evaluation sites.
The work described in this report includes the ongoing monitoring of pasture condition and pasture productivity, with the aim of determining safe stocking rates for the range of pasture-based rehabilitation in the Bowen Basin over a more realistic timeframe. Confidence in an appropriate carrying capacity is a prerequisite to further negotiations towards pasture-based rehabilitation lease relinquishment.
Alongside issues of land capability, the Stage 2 extension has made progress towards evaluating stakeholder attitudes and opinions as a gauge of community acceptance of grazing as an end-use. When the project was conceived, little was known of the social acceptance of pasture-based lease relinquishment. A component of this project was to investigate the perceptions of grazing as a post-mining land-use by various interest groups, with particular emphasis on neighbouring graziers. The report concentrates on the biophysical components of the project, and outcomes of the stakeholder study are reported separately.
The following sections summarise the major findings and conclusions from each of the main components of this stage 2.
The Stage 2 project extended an initial two-year monitoring of three grazing trials that were established under Stage 1. Available forage (dry matter yield), live cover and litter, and species composition were monitored across all trials between February 2000 and November 2004. At Goonyella Riverside, an intensive rotational grazing trial using small paddocks (up to 0.8 ha in size) was designed to create a total of eight different grazing pressures. The trials at Norwich Park and Blackwater mines tested continuous grazing regimes under different stocking rates in paddocks up to 35ha in size. Stocking rate treatments were discontinued in April 2002 at Blackwater, however, pasture condition assessments were maintained for the single stocking treatment. Both trials were designed to validate and expand on the conclusions drawn from the intensive short-term grazing experiment at Goonyella Riverside, and to provide a demonstration of practical management outcomes.
Preliminary modelling of pasture productivity was continued and expanded under Stage 2 in an effort to better define the key variables controlling pasture growth, and hence sustainable carrying capacity, of rehabilitated pastures. Peak pasture yield was measured over four growth seasons in a total of 58 plots at the three mines hosting grazing trials, plus a fourth mine, Saraji. A small number of plots were located on adjacent unmined pasture sites. Peak seasonal pasture growth was analysed in relation to measured site and soil quality parameters.
Conclusions Stage 2
The project has defined the biophysical parameters that allow land managers to make an informed, scientific decision about which sections or parcels of land within the rehabilitated landscapes should be appropriate for supporting a managed grazing regime. Sustainable stocking rates will vary with pasture productivity, which is in turn linked to factors affecting the retention of soil moisture. At the higher end of site productivity such as the Norwich Park trial site, sustainable stocking of at least 3 ha/head could be achieved, and this is equivalent to rates on the better improved pastures on unmined lands in the region. Grazing management will be key to sustainable use, however, and a component of a companion project, C14053 Assessing Grazing Risks for Rehabilitated Pastures in Central Qld, is addressing some of the risks to the landscape documenting the grazing management practices of adjacent graziers.