Ryegrass glyphosate resistance increases

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.