Managing Group A & B
Herbicide Resistance

A tool for managing group A and group B herbicide resistance

Group A and B herbicides are widely used across Australia in many cropping rotations. Extensive use of Group A and B herbicides is increasing herbicide resistance to these modes of action.

Roundup Ready canola growers have greater flexibility with their Integrated Weed Management plan because they can apply Roundup Ready Herbicide with PLANTSHIELD by Monsanto (Group M herbicide) from emergence up to 6 leaf crop stage.

Roundup Ready Herbicide with PLANTSHIELD by Monsanto reduced the reliance on Group A and B herbicides so they can continue to be used in other phases of the cropping rotation.

Increasing instances of Group A and B herbicide resistance

New South Wales and Victoria

A 2013 study by John Broster (CSU) found the following frequency of ryegrass resistance in Southern New South Wales and Victoria:

65
of populations contained plants resistant to clethodim at the 250 ml label rate.
42
of populations also contained plants resistant to clethodim at the higher label rate of 500 ml/ha.

Western Australia

The Australian Herbicide Resistance Initiative (AHRI) recently released the results of a study, which found the following frequency of resistance and wild radish in annual ryegrass populations:

23
of annual ryegrass populations resistant to clethodim. 
2% increase on 2007 survey.
54
of annual ryegrass populations resistant to imazamox/imazapyr. 11% decrease on 2007 survey.
20
of wild radish populations resistant to imazamox/imazapyr.

Managing glyphosate resistance

Over-reliance on glyphosate before, during and after the canola rotation will increase the chance of resistance developing on your farm.


There are a range of herbicides with different modes of action to glyphosate, which can be effectively used within the Roundup Ready canola rotation. Increasing the use of one or more alternative herbicides and integrating other practises will greatly assist in maintaining excellent weed control, while reducing the risk of glyphosate resistance developing on your farm.