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

Abstract

Context New Acland coal mine in south-eastern Queensland is seeking to rehabilitate mined land to pastures that are safe, stable and sustainable for beef production. Little is known of the productivity and sustainability of grazing previously mined land in the Darling Downs study region. Additionally, information is required to specify management guidelines for sustainable grazing of regional land types retired from cultivation. Aims Identify pasture growth characteristics, rainfall use efficiencies and long-term carrying capacities of subtropical sown pastures established on lands rehabilitated after open-cut coal mining in comparison to sown pastures established on un-mined but previously cultivated lands. Methods Pasture growth and quality (% nitrogen) were observed using the Swiftsynd methodology in ungrazed exclosures with three sites on rehabilitated lands of the Acland Grazing Trial over a 5-year period (2014–2018), and 13 sites on unmined lands over periods of 2–5 years providing data for modelling pasture growth. Key results Peak pasture yield (TSDM for autumn harvests) averaged for 2017 and 2018 was greater (P < 0.1) on rehabilitated sites than unmined Poplar Box land type sites (5957 and 2233 kg/ha respectively) but similar to Brigalow Uplands and Mountain Coolibah land type sites (3946 and 3413 kg/ha respectively). Pasture rundown was evident, with pasture N uptake decreasing over 5 years at some sites. Soil mineral N supply (potentially mineralisable N and mineral N) in spring was a useful indicator of N uptake over the following growing season. Simulations using the GRASP pasture growth model for the grazing trial period predicted rainfall use efficiencies of 12.0, 7.0, 9.1 and 4.8 kg/ha.mm rainfall for rehabilitated sites and unmined sites on Brigalow Uplands, Mountain Coolibah and Poplar Box land types respectively. Long-term carrying capacities based on estimates of long-term median pasture growth and 30% utilisation were 4.39, 3.58 and 5.92 ha/adult equivalent respectively for the unmined land types, and 2.45 ha/adult equivalent for the rehabilitated lands. Conclusions Rehabilitated land can be as productive as unmined but previously cultivated land. Implications Grazing management plans for sustainable management of mined and unmined lands can be developed using data from the present study. The plans will assist with the transition of rehabilitated lands to commercial agriculture.