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Resist resistance with double knockdown

Growers can take steps in coming weeks to help keep the precious knockdown herbicide glyphosate working on their farms, according to the Australian Glyphosate Sustainability Working Group (AGSWG).

The AGSWG – a collaborative initiative supported by the Grains Research and Development Corporation (GRDC) – says reducing the number of weeds that growers have to deal with in-crop will help keep glyphosate an effective herbicide.

“To achieve this growers can use tactics such as the double knockdown – two applications of knockdown herbicides used within 10 days of each other – such as glyphosate followed by paraquat,” AGSWG executive officer Andrew Storrie said.

“The aim of the second herbicide is to kill any survivors of the first application. These weeds might be resistant to glyphosate.

“Using measures such as the double knockdown will help local growers avoid the scenario facing their counterparts in the United States and Canada, where an over-reliance on glyphosate has led to big problems with herbicide resistance, with 49 per cent of grain growers surveyed believing they have glyphosate resistant weeds.”

Mr Storrie said more than 20 cases of glyphosate resistance had been confirmed to date in Western Australia’s winter grains systems, and believed many more infestations were going unreported.

He said glyphosate should be applied at the full label rate to help reduce the risk of any weeds surviving it, and sprayed at the right time – in the case of annual ryegrass, when it had reached the two to three leaf stage.

“Annual ryegrass plants that survive being sprayed with glyphosate should be collected and laboratory tested for herbicide resistance, so growers can confirm which herbicides remain effective on them,” Mr Storrie said.

“Herbicide resistance testing can be arranged through your agronomist or local farm supply agent.

“The second knockdown herbicide application should be a robust rate of Spray.Seed® or paraquat but can include the pre-emergent herbicides trifluralin or Boxer Gold®.

“The price of paraquat has dropped significantly, making its use in a double knock strategy much more cost effective.

“Annual ryegrass plants which survive being sprayed with paraquat should be killed using whatever means is possible before seeding the crop.”

Mr Storrie said that if small weeds had not been controlled by an early application of a knockdown herbicide, they would be harder to kill.

“Growers in this situation may have to consider delaying seeding in these paddocks so they can apply a second knockdown herbicide,” he said.

For more information on managing glyphosate resistance visit the AGSWG website

You can download the GRDC Glyphosate Resistance Fact Sheet at, while the GRDC Ground Cover supplement Making Herbicides Last will be included in the May-June edition of Ground Cover. Visit to sign up for or download Ground Cover.

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