Susceptibility of fleabane (Conyza bonariensis) biotypes to glyphosate in northern cotton farming systems of Australia

The rapid emergence of herbicide-resistant weeds has further added complexity to the management of weeds in Australian cotton farming systems. Fleabane (Conyza bonariensis L) has been identified by the cotton industry as being potentially resistant or at risk of developing resistance. Thirty-seven C. bonariensis populations were collected in the 2014-2015 cotton season from 9 cotton farming regions in Queensland and New South Wales. Glasshouse screening trials showed that 27 populations (73%) expressed resistance to glyphosate salt at rate of 0.68 kg a.i ha-1 applied at the rosette stage. Another 9 populations (24%) were categorised as developing resistance. A dose response study of 4 selected resistant populations using glyphosate rate of 0, 0.34, 0.68, 1.36, 2.72, and 5.44 Kg a.i ha-1 showed a high level of Resistance Index (RI) ranging from 3.5 to 7.0 and the dose-response model revealed that 3 to 4 times more glyphosate is required to control resistant C. bonariensis populations. These results further confirm the frequent reports of reduced efficacy of glyphosate against fleabane in northern cotton farming systems of Australia. To reduce the risk of resistance other non-glyphosate tactics are needed in the weed management toolbox.