Heavy grazing of buffel grass pasture in the Brigalow Belt bioregion of Queensland, Australia, more than tripled runoff and exports of total suspended solids compared to conservative grazing

Highlights

• Stocking rate affects ground cover, which affects runoff and erosion.

• Runoff and erosion were measured under conservative and heavy stocking rates.

• Heavy stocking rate more than doubled bare ground.

• Heavy stocking rate more than tripled total runoff and peak runoff rate.

• Heavy stocking rate more than tripled loads of total suspended solids.

Abstract

Loss of sediment and particulate nutrients in runoff from the extensive grazing lands of the Fitzroy Basin, central Queensland, continue to contribute to the declining health of the Great Barrier Reef. This study measured differences in hydrology and water quality from conservative and heavy grazing pressures on rundown improved grass pastures in the Fitzroy Basin. Conservative grazing pressure was defined as the safe long-term carrying capacity for rundown buffel grass pasture, whereas heavy grazing pressure was defined as the recommended stocking rate for newly established buffel grass pasture. Heavy grazing of rundown pasture resulted in 2.5 times more bare ground and only 8% of the pasture biomass compared to conservative grazing. Heavy grazing also resulted in 3.6 times more total runoff and 3.3 times the peak runoff rate compared to conservative grazing. Loads of total suspended solids, nitrogen and phosphorus in runoff were also greater from heavy than conservative grazing.

Introduction

The Fitzroy Basin is Queensland's largest coastal catchment and is almost entirely contained within the Brigalow Belt bioregion of Australia. Both the basin and the wider bioregion have experienced some of the highest rates of land clearing in the world, with up to 93% of vegetation communities dominated by brigalow (Acacia harpophylla) cleared for agriculture since European settlement (Butler and Fairfax, 2003; Cogger et al., 2003; Tulloch et al., 2016). Grazing is the dominant land use in the Fitzroy Basin, with more than 2.6 million cattle over 11.1 Mha (Australian Bureau of Statistics, 2009; Meat and Livestock Australia, 2017a). This is the largest cattle herd in any natural resource management region in both Queensland and Australia, accounting for 25% of the state herd and 11% of the national herd (Meat and Livestock Australia, 2017a).

The 2017 Scientific Consensus Statement for Great Barrier Reef water quality identified the Fitzroy Basin as a high priority area for reducing fine sediment and particulate nutrients. This is due to their ongoing contribution to marine water quality decline and resultant damage to seagrass and coral reefs (Waterhouse et al., 2017). Increased adoption of best management practices for agriculture was identified as a key strategy to reduce sediment and nutrient loads in runoff. Within the Grazing Water Quality Risk Framework for 2017 to 2022, the lowest risk to water quality from hillslope pasture management is achieved by practices such as forage budgeting to determine carrying capacity, ground cover monitoring and the adoption of wet season spelling (The State of Queensland, 2020b). These practices are commonly recommended to maintain or improve ground cover (Jones et al., 2016; Moravek et al., 2017; O'Reagain et al., 2011), as high cover is known to reduce runoff, and hence also sediment and nutrients exported in runoff (Murphy et al., 2008; Nelson et al., 1996; Schwarte et al., 2011; Silburn et al., 2011). For example, light and heavy stocking rates were compared in the Burdekin Basin with 20 to 25% and 40 to 50% pasture utilisation, respectively (O'Reagain et al., 2008). A safe long-term carrying capacity is defined as the capacity of the pasture to sustainably carry livestock in the long-term whereas a safe pasture utilisation rate is defined as the proportion of annual forage growth that can be consumed by domestic livestock without adversely affecting land condition in the long-term (McKeon et al., 2009; Walsh and Cowley, 2011).

In below average rainfall years, the heavy stocking rate had less ground cover, a greater frequency and intensity of runoff, and higher sediment concentrations in runoff. However, there was little difference between the two stocking rates in high rainfall years due to high ground cover (O'Reagain et al., 2008). This reflects international literature from at least the last 100 years that demonstrates heavy continuous grazing accelerates runoff and erosion (Hubbard et al., 2004). Multiple global meta-analyses have shown that grazing decreases ground cover and increases compaction, which consequently decreases protection from raindrop impact, aggregate stability and infiltration while increasing runoff. These impacts were greater from heavy grazing than conservative or rotational grazing (Byrnes et al., 2018; Eldridge et al., 2016; Lai and Kumar, 2020; McDonald et al., 2019; Sirimarco et al., 2018; Wang and Tang, 2019; Xu et al., 2018). Thus, erosion and sediment transport are primarily associated with high-density stocking and/or poor forage stands on grazed landscapes (Hubbard et al., 2004). Globally, degradation by overgrazing is estimated to effect 20 to 35% of permanent pastures, which total about half of the earths terrestrial surface (Byrnes et al., 2018; Lai and Kumar, 2020).

Although spelling pasture has been shown to increase biomass, seasonal conditions can actually have a stronger effect on ground cover and pasture biomass (Jones et al., 2016). This further highlights the importance of managing grazing pressure to maintain landscape resilience, particularly during periods of below average rainfall (Edwards, 2018). Managing grazing pressure is typically undertaken by varying stocking rate, as it is the most powerful management tool available to the grazier (Lawrence and French, 1992).

These interrelated land use and land management issues were a focus of the Reef 2050 Water Quality Improvement Plan (The State of Queensland, 2018). This plan seeks to improve Great Barrier Reef health and resilience by facilitating increased adoption of lower risk land management practices to achieve specific water quality targets. Progress towards these targets is measured via the Paddock to Reef Integrated Monitoring, Modelling and Reporting program (Paddock to Reef program) (Waterhouse et al., 2018). The Paddock to Reef program is underpinned by a modelling framework that ranges in scale from individual paddocks though to entire basins with real-world validation provided by numerous studies (Waterhouse et al., 2018). The Brigalow Catchment Study is a paddock scale study that is used to validate the effects of hillslope grazing management on water quality from the Fitzroy Basin. This long-term study has a paired catchment design where catchments are adjacent within a uniform landscape, whereas other paired catchment studies often have sites located further apart in the landscape which confounds interpretation due to inherent differences in soil, slope, vegetation and climatic sequences.

Despite the existence of about 200 paired catchment studies world-wide (Peel, 2009), only 13 of them are based in Australia and only three of these have any form of pasture treatment (Best et al., 2003). Two of the three pasture studies were based in Mediterranean climates (cool wet winters and hot dry summers) and are now both inactive (Best et al., 2003; Mein et al., 1988), whereas the third at the Brigalow Catchment Study was based in a semi-arid, subtropical climate (warm wet summers and cool dry winters) and remains active. Bartley et al. (2012) noted that there is a limited amount of Australian runoff and water quality data that is urgently required for modelling activities, such as determining progress towards achieving the Reef 2050 Water Quality Improvement Plan targets. This study provides empirical data from the Fitzroy Basin to determine the effects of grazing management practices on paddock scale water quality. More specifically the study aims to:

• Quantify the impact of conservative and heavy cattle grazing pressures on hydrology and both event mean concentrations (EMCs) and loads of total suspended solids, nitrogen and phosphorus in hillslope runoff over four hydrological years (2015 to 2018);

• Determine the anthropogenic impact of cattle grazing by comparing hydrology and both EMCs and loads of total suspended solids, nitrogen and phosphorus in hillslope runoff from a conservatively grazed pasture and virgin brigalow woodland which is representative of the pre-European landscape; and

• Quantify the impact of conservative and heavy cattle grazing pressures on pasture biomass and ground cover over four hydrological years (2015 to 2018).