Testing times for weed resistance

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Portrait of Brogan Micallef, Prof Steve Powles and Prof Mark Gibberd at a conference

(From left) Brogan Micallef, of AHRI, AHRI director Professor Stephen Powles and CCDM director Professor Mark Gibberd.

PHOTO: Cox Inall 

Despite growing evidence of escalating levels of glyphosate resistance in Western Australia, creative management will ensure the sustainability of this vital Group M herbicide.

That is the advice of Australian Herbicide Resistance Initiative (AHRI) director Professor Stephen Powles, who spoke at the inaugural GRDC-supported Crop Protection Forum in Perth in November 2015.

This forum was coordinated by AHRI and the Curtin University-based Centre for Crop and Disease Management to bring together leading weed and pesticide-resistance researchers.

Professor Powles told participants that glyphosate is still working well on the majority of WA farms, but stressed that growers and advisers need to diversify its use with other herbicide modes of action and non-herbicide weed-control tactics to preserve its effectiveness in the long term.

“As I keep emphasising, diversity is the key and when on a good thing, don’t stick to it,” he said.

Tips for testing weeds for herbicide resistance

  • Testing services include Plant Science Consulting’s Weed Resistance Quick Test.
  • Weed samples should come from
  • high-risk paddocks or be taken from
  • areas where weeds have survived a herbicide application.
  • Samples should include about 50 to
  • 100 plants per herbicide for a four-herbicide analysis.
  • Samples are best collected before applying an in-crop herbicide, at less than five to six-leaf stage for broadleaf species and at one to two-leaf stage for grasses.

Random surveys of WA grainbelt paddocks by AHRI researcher Mechelle Owen in 2010 found seven per cent of 362 annual ryegrass (Lolium rigidum) samples had glyphosate resistance. This was up from one per cent in 2003.

Further herbicide resistance surveys conducted by AHRI in 2010 and 2011 in 239 paddocks of Roundup Ready® canola across WA found 3.3 per cent of annual ryegrass samples were glyphosate-resistant.

In these surveys, there was no glyphosate resistance found in wild oats (Avena), brome grass (Bromus) or barley grass (Hordeum).

In 2013, as part of a glyphosate, paraquat (Group L) and 2,4-D (Group I) resistance project, Department of Agriculture and Food, WA (DAFWA) senior research officer Dr Sally Peltzer coordinated a targeted pre-harvest survey across WA’s cropping areas to raise awareness about the spread of glyphosate resistance in the state.

She found 40 per cent of 172 samples of annual ryegrass from more than 150 farm properties had some glyphosate resistance.

Of these, 32 per cent had weak (or developing) resistance, eight per cent had intermediate resistance to the lower glyphosate testing rate and 11 per cent had high-level resistance.

It should be noted that the DAFWA survey targeted growers who had weedy paddocks, compared with AHRI’s random sampling.

In 2013, AHRI researchers uncovered Australia’s – and the world’s – first cases of glyphosate-resistant wild radish (Raphanus raphanistrum L.) populations in WA’s northern grain-growing region.

Professor Powles said random surveys in South Australia and New South Wales also revealed an increasing incidence of glyphosate resistance – to levels as high as 16 per cent in annual ryegrass in south-east SA (2012) and one to two per cent on the NSW Slopes region (2013).

He said these findings are consistent with analyses of weed samples sent to Australian herbicide-resistance testing services at Charles Sturt University and Plant Science Consulting.

Professor Powles said across Australia most glyphosate resistance is found where the herbicide is used persistently.

However, he said the good news is that it is still highly effective where growers are diversifying their weed-control practices.

Best-practice integrated weed management tactics are outlined on the national grains industry’s WeedSmart campaign website.

Professor Powles said growers who find herbicide ‘survivors’, or weedy patches, on their farms this year can send samples for a resistance test.

He said if resistance is confirmed, growers should act immediately and not allow the resistant weeds to set seed.

More information:

Professor Stephen Powles,
08 6488 7834,

Useful resources:

Integrated Weed Management Hub
Glyphosate resistance fact sheet

In-crop herbicide use fact sheet
Ground Cover Supplement herbicide resistance
Australian Herbicide Resistance Initiative website 
Australian Glyphosate Sustainability Working Group

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