Dense canopy effective against sowthistle

A GRDC-funded survey conducted across the GRDC Northern Grains Region in 2014–2016 discovered that glyphosate resistant sowthistle was more widespread than previously thought. Originally identified on the Liverpool Plains of NSW in 2013–14, resistant populations have now been identified throughout the region, from central Queensland to central NSW.

“Currently, it seems that many resistant populations are exhibiting relatively low level resistance,” said Dr Annie van der Meulen. “This means glyphosate is still effective in many situations however attention to detail will make a difference to application efficacy.

“Plant size is very important when applying glyphosate to sowthistle,” says Dr van der Meulen.

If a lower rate is applied to larger weeds they can take a long time to die (if at all) and may well set seed before the plant dies, making the application a waste of money.

To stay in control of sowthistle growers need to be ready to respond to new flushes at any time that surface soil moisture is available. One weakness that growers can exploit is that sowthistle is a poor competitor, so choosing crops that rapidly achieve canopy closure can be a very effective control measure.

To gain a deeper understanding of the extent of herbicide resistance, GRDC has funded further surveillance work in the northern grains region. As part of this work DAF is asking growers to give permission for weed samples to be collected. Contact Adam Jalaludin at DAF on 13 25 23. A similar survey, managed by John Broster at Charles Sturt University, will be conducted in NSW.

What are the control options for sowthistle in the fallow?

Target small, actively growing plants with robust rates of glyphosate. Use optical sprayer technology and the maximum label rate. Double-knocking treats survivors and provides control for cohorts that germinated after the first spray. 

Are there any control options in-crop?

Crops that form a dense canopy, and close over quickly, will help suppress most weeds. Winter cereals, such as barley and narrow-sown wheat, can provide very effective control of sowthistle. 

What can I do at harvest to control sowthistle?

Plants cut by the harvester can re-shoot, so control established plants as soon as possible following harvest, and be ready to spray new flushes when the plants are small.