NEW research has shown that trifluralin sprayed with higher water rates of about 100L/ha improves the herbicide’s effectiveness on paddocks with heavy stubble loads, regardless of the spraying speed.
The trial results are timely for Western Australian growers given favourable seasonal conditions in 2011 have resulted in high levels of stubble biomass, which can reduce the efficacy of pre-seeding herbicides.
The research was funded by the Grains Research and Development Corporation (GRDC), the WA No Tillage Farmers Association (WANTFA) and Wheatbelt Natural Resource Management (NRM), and conducted by the Department of Agriculture and Food (DAFWA), WANTFA, Wheatbelt NRM and the Australian Herbicide Resistance Initiative (AHRI).
It builds on previous GRDC funded work by DAFWA weeds researcher Catherine Borger, which showed that increasing the water rate improved the performance of trifluralin in minimum tillage systems.
"However, due to equipment limitations, the previous research was conducted at speeds often below those that growers would use to spray trifluralin,” Dr Borger said.
“The current research used normal spraying speeds of 22-24 km per hour and resulted in similar improvements in trifluralin effectiveness, showing that the impact of high water rates on trifluralin performance is not affected by spraying speed.”
Dr Borger said that in trials conducted at Cunderdin and Wongan Hills in 2011, the amount of annual ryegrass which survived 2.5L/ha of trifluralin decreased from 19 to 14 per cent when the water rate was increased from 50 to 100L/ha.
“This was probably because the spray coverage on the ground increased from an average of 9 to 26 per cent when water rates were doubled,” she said.
“Both trials at Cunderdin and Wongan Hills in 2011 were sprayed at normal speeds that growers might use on-farm – 22 to 24km/h - in relatively windy conditions.
“However, the trifluralin was applied using coarse spray quality to cause less drift, at pressures of 3 to 5 bar to give high droplet speed.”
Dr Borger said research conducted by AHRI researcher Mechelle Owen under the same project had shown that trifluralin resistance in annual ryegrass was still at fairly low levels in WA.
Analysis of a 2010 herbicide resistance survey had revealed that about 25 per cent of annual ryegrass populations had low levels of resistance, where 1 to 20 per cent of plants might survive.
“These results show that trifluralin should still work well in most areas of WA, as long as it is applied correctly, and that cases of reduced effectiveness will usually be due to lower water rates on heavy stubble loads, not resistance,” Dr Borger said.
She encouraged growers to check the label rate when planning pre-emergent herbicide application, and said trifluralin was most effective when used after a good rainfall event, and should be incorporated directly after spraying.
