Glyphosate-resistant annual ryegrass populations have been confirmed at 87 sites across Australia, prompting a warning to growers from weed expert Dr Chris Preston of the University of Adelaide.
Dr Preston, whose work on managing glyphosate resistance in weeds is supported by growers and the Australian Government through the Grains Research and Development Corporation (GRDC), said glyphosate resistance had also recently been confirmed in two other weed species in Australia.
"Three populations of awnless barnyard grass and two populations of liverseed grass have been confirmed resistant to glyphosate in a summer cropping/fallow situation in northern New South Wales," he said.
"Resistance in these summer grass weeds has major implications for the management of summer fallows and weed control in summer crops.
"It also demonstrates that weeds other than annual ryegrass can evolve resistance to glyphosate."
Dr Preston said glyphosate resistant annual ryegrass occurred when populations were treated intensively with glyphosate, where no other herbicides were applied and where there was little or no tillage.
"Relying solely on glyphosate for weed control is the greatest risk factor for glyphosate-resistant weeds," he said.
"So far two mechanisms of resistance have been discovered – target site mutation (a mutation in an annual ryegrass enzyme targeted by the herbicide) and reduced herbicide translocation (a reduction in the ability of the herbicide to move inside the plant).
"Populations of annual ryegrass can accumulate both mechanisms of resistance and are much more resistant to glyphosate.
"Therefore, continued use of glyphosate on resistant populations may result in higher resistance.
"To date, glyphosate resistance has not occurred widely in no-till systems despite the large amount of the herbicide being used in this system.
"Studies have indicated that glyphosate-resistant populations of annual ryegrass to not perform well under crop competition.
"However resistance can appear in areas with little competition, like fence lines, and be dragged into the cropped area with harvest and seeding equipment creating a problem in the paddock."
Significantly, Dr Preston said 20 populations of glyphosate-resistant annual ryegrass had so far been confirmed at fence lines from Western Australia to NSW.
Another 11 populations had been confirmed in other uncropped areas such as irrigation channels and firebreaks.
"Fence lines offer fewer options for controlling glyphosate-resistant annual ryegrass," he said.
"Tillage and competition are more difficult to employ, so herbicides will likely remain the main option.
"Our studies have shown glyphosate and glyphosate mixes do not provide effective control.
"Current research indicates paraquat mixed with amitrole or diuron provided better control of both resistant and susceptible annual ryegrass.
"Growers should use robust rates of products on fence lines to ensure control."
Dr Preston said a survey of the area around Port Broughton on South Australia’s upper Yorke Peninsula had found six paddocks with Indian hedge mustard resistant to Group I herbicides.
The population is already resistant to all Group B herbicides.
"This is a worrying development as Group I herbicides are often used to control Group B-resistant broadleaf weeds," he said.
"At the moment it probably isn't widely spread but a more extensive survey is underway to determine that for sure."
