Resistance prevention in weeds 'impossible'

Experts have warned grain farmers to use a minimum of three different strategies each season to manage their weeds, or face up to 50 per cent yield losses because of resistance issues.

Following the discovery of annual ryegrass resistant to both glyphosate and paraquat in a vineyard in the South West, graingrowers have been urged to test for herbicide resistance and manage any problems before they impact on long-term profitability.

The finding was made in a WA vineyard after Department of Agriculture and Food senior researcher Sally Peltzer was alerted to the problem by the vineyard owner.

"It isn't just vineyard managers who need to be having a serious look at how they use knockdown herbicides, as it raises concerns about broadacre management practices as well," she said.

While the problem first appeared to be a paraquat failure, treating samples with glyphosate showed the ryegrass was failing to respond to that knockdown herbicide as well.

Australian Glyphosate Sustainability Working Group executive officer Andrew Storrie said it was now time to forget about resistance prevention.

"We've gone beyond prevention - the only way to manage resistance is to ensure that survivors of any herbicide application are killed by some other means to chemicals," he said.

"That may mean pulling the weeds out by hand, intensive grazing or even coming in with a mower and slashing."

Mr Storrie said the discovery of the double resistance weeds was clear evidence that rotating modes of chemical action simply delayed the inevitable.

"All rotating modes of action does is simply delay the development of resistance, and this case gives us an example that instead of taking 12 or 15 years to happen, it's taken 20, but we've now got the added problem of multiple resistance," he said.

"The real message that has come out of this whole issue is that some farmers and other land managers think they are doing integrated weed management simply by having rotated herbicide modes of action, which isn't enough."

Mr Storrie said growers must also consider a double knock prior to seeding to get on top of resistant populations.

"You come in first with your glyphosate, then you kill any survivors with the paraquat application," he said.

"A lot of farmers don't like to do this because it means going over the paddock twice."

Mr Storrie said harvest weed seed management strategies were also essential.

"Everybody is going to have to do some sort of harvest seed management on basically every paddock sooner than we originally thought, whether it s chaff cart, narrow windrow burning or the Harrington Seed Destructor, that's going to be a given," he said.

"We've been saying it's a big issue for some time now but human nature is that you don't do anything until you have a problem.

"One of the other problems is that a lot of people see that it's too big to deal with, but we need to start looking at the little things to get on top of it."

GRDC Western Region Panel chairman Peter Roberts reiterated the call to increase diversity in weed management.

"Diversity is the only answer," he said.

"Relying on one mode of action, even two modes of action is probably going to cause us some headaches in the future."

Mr Roberts said weeds were potentially the biggest threat to production in the western region's cropping system, but paraquat resistance issues were not new, having been discovered already in South Australia and South Africa.

"Seeing the first population of ryegrass being resistant to paraquat in WA is really just ringing the bell and telling farmers this can happen," he said.