Australia tops world in herbicide resistance

Australia has the world's most severe herbicide resistance problem, Western Australia's Prof Stephen Powles told delegates at the XIX International Congress of Genetics on Monday.

"It's by far a bigger issue in Australia than in any other country in the world," said Powles, who leads the Western Australian Herbicide Resistance Initiative.

At the heart of the problem is the widespread planting of annual ryegrass, Lolium rigidum, as pasture grass for sheep, an industry around since the mid-1800s. But when the bottom fell out of the livestock industry a couple of decades ago, crops rapidly took over as the major form of agriculture in areas previously devoted to sheep farming.

"All of a sudden pasture grass became a weed, a target for chemical control," Powles said. But Lolium has high genetic variability and is present in high density across a large swathe of the country, a recipe for disaster when combined with use of herbicides.

"99.5 per cent of crop fields in Australia are herbicide treated," he said.

The result has been rapid development of multiple herbicide resistance. Powles has demonstrated that the resistance to the two major classes, ALS herbicides and ACCase herbicides, was due to mutations in single semi-dominant genes for target-site based resistance.

Other mechanisms of herbicide resistance involve P450 genes, and several are implicated in blocking the action of other classes of herbicides. Many varieties have also accumulated multiple resistance genes. And a big worry for farmers is the appearance of resistance in Australian Lolium to glyphosate for the first time in the world.

Powles said that farmers only have themselves to blame for the problem. "We're not that smart. We created and nurtured it ourselves," he said.