The Brigalow Belt Bioregion of Queensland has been extensively cleared for agriculture, with the majority of cleared areas being utilised for grazing (Thornton and Elledge 2013). If left uncontrolled, the proportion of woody species in grazed pastures increases over time, leading to a decrease in pasture productivity (Burrows 2002, Scanlan 2002). This productivity decline has negative implications for both the environment and the viability of grazing enterprises. In order to avert productivity decline, the regrowth of woody species in grazed pastures is typically controlled by either mechanical intervention, such as blade ploughing or stick raking, or by the use of herbicides, such as tebuthiuron (Partridge et al. 1994).
Under Objective 1 of Reef Plan 2009 (Department of the Premier and Cabinet 2009), tebuthiuron is one of the five priority herbicides identified as main pollutants in water entering the Great Barrier Reef lagoon. A literature review of tebuthiuron research in Australia found 12 published papers; six of which focused on end of system water quality loads and concentrations at the basin scale (>10,000 km2) with little focus on process understanding. Thus, this study sought to examine the movement of tebuthiuron in runoff at both the plot (1.7 m2) and paddock (12.7 ha) scales, in addition to movement in soil.
At the paddock scale, runoff 100 days after application showed high concentrations of tebuthiuron (mean 103 µg/L); however, the total lost in runoff was only 0.05% of the amount applied. Runoff was monitored up to 472 days after application with losses of applied tebuthiuron ranging between 0.05 and 0.45%. The highest loss was associated with the highest discharge runoff event. Comparison of individual event hydrographs showed that tebuthiuron concentrations declined exponentially with time. Furthermore, results indicated that tebuthiuron movement in runoff was in a dissolved form rather than being transported adsorbed to suspended sediments.
At the plot scale, maximum load of tebuthiuron in the soil from 0 to 5 cm deep occurred 28 days after application. This is likely due to the movement of tebuthiuron from trash into the soil following rainfall. Maximum loads in the soil from 5 to 20 cm deep occurred between 56 and 98 days after application. After 98 days tebuthiuron concentrations at all depths tended to decrease over time. Losses of tebuthiuron in runoff were greater from Vertosols than Sodosols.
This study has filled a significant knowledge gap about the behaviour of tebuthiuron at the plot and paddock scale, and provides data where none previously existed. The data also improves understanding of tebuthiuron behaviour from predominantly grazed catchments that discharge into the Great Barrier Reef lagoon. Limitations of this study are: (1) data were sourced solely from the Fitzroy Basin; (2) only Vertosol and Sodosol soil types were investigated; (3) investigation of persistence in soil was limited to particular profile depths and time intervals; and (4) runoff at the paddock scale was limited to 17 months data with only small runoff events immediately after application. These limitations could be easily addressed by replicating the plot components of this study on additional soil types in other Great Barrier Reef catchments. Opportunities also exist for replicating the paddock scale work in additional catchments of the Brigalow Catchment Study.