Growers in herbicide sustainability ‘mode’

Author: Rachel Bowman | Date: 26 Jul 2013

Dr Chris Preston, University of Adelaide – Associate Professor, Weed Management

Landholders across Australia are being urged to get to know the standard modes of action (MOA) for herbicides as a key strategy for protecting against the evolution of resistant weed populations.

Dr Chris Preston, University of Adelaide Associate Professor – weed management says understanding and rotating herbicide MOAs is crucial for protecting current herbicides.

“New herbicide chemistries are rare so growers should do everything they can to protect them,” Dr Preston said.

“This means not relying on the same mode of action every year to control weeds.

“The last new herbicide mode of action was introduced into Australia in 1996. Since then all new active ingredients introduced belong to existing herbicide modes of action.”

In 1996, a change was made to herbicide labels requiring the MOA to be placed on the label.

The letter codes were updated in 2008 and now all products sold in Australia carry a letter on the label indicating the MOA. A total of 19 letter codes divide herbicides into MOAs and chemistries.

Dr Preston says Group A and Group B herbicides carry the highest risk of resistance.

“Group A herbicides are only active against grass species and are often known as graminicides,” he said.

“These herbicides are widely used to control grass weeds in crops. To date 10 grass species have resistance to Group A herbicides in Australia.”

Dr Preston says Group B herbicides are some of the most successful crop herbicides used worldwide. In Australia, they are used in many crops including Clearfield® crops, in fallows, and in non-agricultural application.

“Two negative features of Group B herbicides are their extended persistence in some soils, leading to damage to subsequent crops or to desirable native species, and the rapid emergence of herbicide resistance to this MOA.

“Currently there are seven grass species and 17 broadleaf species with resistance to Group B herbicides.”

Dr Preston says while rotation of herbicide MOA is an important tactic for resistance management, it needs to be coupled with other with other tactics, such as weed seed set control to obtain the benefit.

“By understanding the MOA herbicides they are using, growers can make better decisions about herbicide use,” he says.

Dr Preston has lent his expertise to the WeedSmart campaign which aims to galvanise the Australian cropping industry against herbicide resistance.

WeedSmart involves industry organisations including GRDC, research providers and major crop input firms to deliver the message that key changes need to occur on-farm to halt the spread of resistance.

For more detail on herbicide MOAs and groups, visit For more information on herbicide sustainability practices, visit or


PHOTO CAPTION: Dr Chris Preston, University of Adelaide says growers should get to know herbicide modes of action as part of their herbicide resistance management plans.

For interviews contact:

Dr Christopher Preston
University of Adelaide – Associate Professor, Weed Management
School of Agriculture, Food & Wine
Ph: (08) 8313 7237


Rachel Bowman
Cox Inall Communications
Ph: 07 3846 4380
Mobile: 0412 290 673


WeedSmart is an industry-led initiative aimed at enhancing on-farm practices and promoting the long term sustainability of herbicide use in Australian agriculture. Australian research partners, commercial entities, Government, advisers and growers have joined forces to ensure weed management remain at the forefront of global farming practise. Viable herbicide use will help secure the weed control productivity gains made by the current generation of Australian farmers.

Region North