Sustainability of beef production from brigalow lands after cultivation and mining. 3. Pasture rundown, climate and grazing pressure effects

Abstract

Context. The Acland Land System overlying the Walloon sandstone coal deposits in southern Queensland is generally marginal for cropping but well suited to grazing, and thus cultivated land is commonly returned to pasture. Rehabilitation of these lands after open-cut coal mining seeks to be safe, stable and self-sustaining to satisfy requirements for ecologically sustainable development.Aims. The present paper evaluates the sustainability and economic viability of beef production on (a) lands retired from cultivation and then rehabilitated with sown pastures after open-cut coal mining at the New Acland mine site, and (b) similar nearby pasture lands that were not mined but were also retired from cultivation.Methods. The GRASP grazing systems model was modified and calibrated with short-term (5-year) grazing trial data (soil, pasture and cattle observations), and then used with long-term (60-year) weather data to estimate effects of land type, pasture rundown, climate and grazing pressure on productivity and economic returns. The productivity of three rehabilitated sites and 15 unmined sites were evaluated, including pastures on six commercial properties.Key results. Estimates of long-term mean annual growth of pastures on unmined lands retired from cultivation on three land types (Mountain Coolibah, Brigalow Uplands and Poplar Box) were 3398, 2817 and 2325 kg/ha respectively. Pasture growth was greater on rehabilitated lands; 3736 kg/ha on the site most typical of rehabilitated lands and a mean of 4959 kg/ha across three sites. Seasonal conditions had large effects on cattle liveweight gain (133-213 kg/head per year during the trial); however, pasture growth was the main driver of beef production and economic returns per hectare. In GRASP, potential nitrogen uptake was used to influence key pasture growth processes and accounted for 64% of variation in observed annual growth. The short-term lift and subsequent rundown in productivity typically associated with sown pastures was estimated to have increased mean annual pasture and cattle productivity during the 2014-2018 trial period by up to 17% and 25% respectively. Estimates of long-term mean annual beef production and economic returns for the unmined lands were less than estimated for rehabilitated lands and were 139 kg/head.year (45 kg/ha.year) and AU$154/adult equivalent.Conclusions. Rehabilitated lands were found to be sustainable for beef production at grazing pressures up to 30% utilisation of annual pasture growth, and comparable with grazing systems on native and sown pastures in good condition. Pastures on unmined lands retired from cultivation had reduced productivity.Implications. Overgrazing is a significant and on-going residual risk to sustainable production. Grazing regimes need to continually adjust for changes in novel landscapes, pasture condition and climate. The methods used in the present study could be applied more generally.

Sustainability of beef production from brigalow lands after cultivation and mining. 1. Sown pasture growth and carrying capacity

“…Three sites on commercial beef properties (Colliery Park and Roundview) were measured for 3 years (2013)(2014)(2015) and 13 sites were associated with the Acland coal mine as follows: (1) five sites adjacent to the mine were in paddocks used for the Acland Grazing Trial and were measured each year for the 5-year period 2014-2018, and (2) eight sites known as the benchmark sites (BMK) were on unmined lands within 7 km of the mine and were measured for 2 years (2017)(2018). Data from the Swiftsynd observations together with additional soil, pasture and cattle data associated with the grazing trial and BMK sites (Bennett et al 2021;Melland et al 2021) were then used to calibrate the GRASP model (McKeon et al 2010;Clewett et al 2021), assess the effects of climate and derive estimates of long-term (60-year) mean annual pasture growth, RUE and LTCC.…”

“…The GRASP model (McKeon et al 1990(McKeon et al , 2000 modified for sown pastures (Clewett 2015;Clewett et al 2021) was used to estimate daily changes in the soil water balance, pasture growth, N uptake and TSDM of pasture for all pastures where Swiftsynd exclosure sites were located. The model was first calibrated to the field observations of TSDM in the Swiftsynd exclosures by using the root mean-square of differences (RMSD) between model estimates and observed values as the objective function.…”

“…The model was first calibrated to the field observations of TSDM in the Swiftsynd exclosures by using the root mean-square of differences (RMSD) between model estimates and observed values as the objective function. The model was further calibrated to soil, pasture and cattle observations recorded for the Acland Grazing Trial and this resulted in some very minor changes to the model's pasture parameters (Clewett et al 2021). It was concluded that the Swiftsynd sites adequately represented pastures in the surrounding grazed paddocks.…”

“…The research reported here is integral to the Acland Grazing Trial (Bennett et al 2021;Clewett et al 2021;Melland et al 2021) established in 2013. The aim was to assess the sustainability and economic viability of beef cattle production from lands that were rehabilitated after open-cut coal mining, compared with surrounding unmined lands (Newsome et al 2014).…”

“…Seasonal pasture growth is measured in relation to soil type, fertility, depth, moisture holding characteristics and daily rainfall, for a minimum of 1 year. Data are used to calibrate the GRASP pasture growth model (McKeon et al 2000(McKeon et al , 2010Clewett et al 2021) for a land type and climate location, and the model is then run using long-term historical weather data to generate average annual pasture growth, rainfall use efficiencies (RUE) and safe utilisation rates necessary for calculating LTCC.…”