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Rotate don’t repeat chemicals

Repeated application of herbicides with the same mode of action (MOA) is the single greatest risk factor for the evolution of herbicide resistance.

That’s the message two of Australia’s leading herbicide resistance researchers, Dr Chris Preston and Dr Stephen Powles, have joined forces to drive home to primary producers and advisers this winter cropping season.

The researchers agree the key to herbicide sustainability is to rotate MOAs and implement diverse chemical and non-chemical tactics, including harvest weed seed control practices to prevent survivors putting seed into the seedbank.

Dr Preston, University of Adelaide, Associate Professor – weed management says
rotation of herbicides is important but alone will not stop resistance, just delay it.

“Rotation of chemicals and MOAs needs to be used in conjunction with practices that reduce weed populations, such as seed set management practices,” he said.

“But it is vital for growers to protect the existing herbicide resource as the discovery of new, effective herbicides is rare.

“There is no quick chemical fix on the horizon so we advise growers to use breakcrops where suitable to manage disease and weed burdens.”

Dr Preston says despite the hundreds of individual products and the large number of active ingredients available for use, most herbicides can be grouped by chemical similarity into relatively few chemical groups.

“Often several herbicide chemistries will have similar biochemical effects on the plant and when these actions of herbicides are considered an even fewer number of modes of action occur.”

He says growers should become familiar with these herbicide modes of action and rotate them whenever practical.

“A good example is the new pre-emergent herbicides Sakura® and Boxer Gold®.

“Sakura® can only be used in wheat (not durum) and triticale; Boxer Gold® can be used in wheat and barley, so they should be used in those crops with trifluralin in other crops to form the basis of a MOA rotation.”

Dr Powles, Australian Herbicide Resistance Initiative (AHRI) Director and University of Western Australia Winthrop Professor hosted Australian and international scientists at the recent Global Herbicide Resistance Challenge.

He says rotation of chemicals and groups is important for protecting existing herbicides and urges landholders to adopt this practice along with the all-important harvest weed seed control techniques promoted by AHRI.

Dr Powles says there is a common misconception amongst producers that a quick chemical fix is on the horizon.

“There are many herbicide product retail names on the market but growers can always know and record the MOA because it is mandatory in Australia that herbicides display the MOA by alphabetical symbol on the herbicide container,” he said.

The recommendation to rotate chemicals and groups is an important part of the WeedSmart campaign which aims to galvanise the Australian cropping industry against herbicide resistance.

WeedSmart brings together industry organisations including the Grains Research and Development Corporation (GRDC), research providers and major crop input firms to deliver the message that herbicide resistance is a difficult but not insurmountable problem – but changes need to occur on-farm.

For more information on herbicide sustainability practices, visit

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