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)(2015)(2016)(2017)(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.

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

“…Parameters in GRASP were calibrated to the soil, pasture and cattle observations on rehabilitated (rehab) and unmined land of the Acland Grazing Trial. Methods for these observations are fully described in companion papers by Bennett et al (2021), Paton et al (2021) and Melland et al (2021) and are summarised below to provide adequate context for the present paper.…”

“…Most unmined sites were retired from cultivation to pasture in the period of 2003-2012, as shown in Table 1. Rhodes grass (Chloris gayana) and creeping blue grass (Bothriochloa insculpta) were the most frequently encountered species across sites (Clewett 2015;Melland et al 2021;Paton et al 2021). Some notable variations in species composition and condition included the following: Gatton and green panic (Megathyrsus maximus) in the Rehab 1 and 2 paddocks, Gatton panic at Colliery Park, high proportions of Queensland blue grass (Dichanthium sericeum) in BMK sites 3 and 16, buffel grass (Cenchrus ciliaris) in the pastures at Oakleigh and Canimbla, couch grass (Cynodon dactylon) exceeding 95% of the pasture at Roundview, and winter active legumes such as medics (Medicago spp.…”

“…Pasture observations of total standing dry matter (TSDM) were made within ungrazed fenced exclosures (12 m • 12 m) by using the Swiftsynd methodology (Day and Philp 1997) for (a) 3 years (2013)(2014)(2015) at the Colliery Park and Roundview sites, (b) 5 years (2014)(2015)(2016)(2017)(2018) at sites within the rehab and control paddocks and (c) 2 years (2017-2018) on the eight benchmark sites (Paton et al 2021). The exclosures were mown to ground level in September-October, at the start of the growing season in spring each year.…”

“…Grazing management on all properties varied with seasons and all employed periods of pasture spelling. Methods used to calculate statistically significant differences in pasture and cattle observations are given by Paton et al (2021) and Melland et al (2021).…”

“…Clewett et al 2018). Field observations from the trial are described in three papers regarding the soils, pastures, grazing system and beef production (Bennett et al 2021;Paton et al 2021;Melland et al 2021). The grazing trial was established in 2013 by the Acland Pastoral Co. on land that had been previously used for dairy, beef and crop production before open-cut coal mining began in 2002.…”