Optimising Group A herbicides in canola

Group A herbicides were first commercialised in Australia in the early to mid 1980s.

In the absence of herbicide resistance, these herbicides quickly and effectively shut down growth of small grass weeds. This removes competition within days of application, despite often taking a number of weeks for herbicide symptoms to become visible.

Several different Group A herbicides are used in Australia. Those that are registered for use in broadleaf crops such as canola are from the aryloxyphenoxypropionate (fop) or cyclohexanediones (dim) sub-classes. Other Group A herbicides used in certain cereal crops are not covered in this Fact Sheet.

Key Points

• Herbicide resistance in winter grass weeds is compromising performance in many paddocks.
• Conduct a resistance test to know which Group A herbicides are still effective.
• Correct application timing is critical for maximising efficacy while minimising potential for crop injury and residues in grain. Target small weeds, early in the crop.
• Environmental conditions can affect herbicide performance, especially frost prior to herbicide application.

Useful Resources

• Congreve, M. and Cameron, J. (2018). Understanding post-emergent herbicide weed control in Australian farming systems - a national reference manual for agronomic advisers. GRDC publication, Australia. https://grdc.com.au/understanding-post-emergent-herbicide-weed-control
• Grain Orana Alliance (2017). The impact of rate and timing of clethodim herbicide applications on canola performance (4 trial reports). http://www.grainorana.com.au/documents
• Saini, R., Boutsalis, P., Dolman, F., Gill, G., Preston, C. (2015). Maximise clethodim performance: impact of frost http://www.weedsmart.org.au/wp-content/uploads/2016/01/Clethodim-and-Frost-Factsheet-2015.pdf
• Saini, R., Malone, J., Preston, C., Gill, G. (2016). Frost reduces clethodim efficacy in clethodim-resistant rigid ryegrass (Lolium rigidum) populations. Weed Science 64(2):207-215 https://bioone.org/journalArticle/Download?fullDOI=10.1614%2FWS-D-15-00140.1
• Zerner, M. and Wheeler, R. (2013). Clethodim tolerance in canola. SARDI trial report. https://www.farmtrials.com.au/trial/16989