Waterhemp Assault: It’s Time to Rethink Weed Control

LeRoy Toohey

03 Mar 2019

As difficult as it is to control waterhemp today, it may be even harder to control in the future. This is one of the many reasons that it may be time to rethink weed control. We're looking at waterhemp as an example weed, and what you can do to get it (and other weeds) under control on your farm. 

What do Group 15 herbicides do?

Group 15 herbicides are long-chain fatty acid inhibitors that are used for pre- and post-residual herbicide applications. They are used to prevent waterhemp and other weeds from emerging and provide little to no control when used as a foliar application.

Some common names of the Group 15 herbicides include: Dual Magnum® (s-metolachlor), Harness®, Warrant®, Degree®, Surpass® (acetochlor), Zidua® (pyroxasulfone), and Outlook® (dimethenamid-p).

How did herbicide resistance develop?

The University of Illinois has documented Group 15 resistant waterhemp in 2 Illinois counties, and waterhemp is the first broadleaf weed to develop resistance to Group 15 herbicides worldwide. Four other cases of resistance to Group 15 herbicides were all grasses, with one of those being in the U.S.

Waterhemp has proven its ability to evolve to tolerate different herbicides. In fact, this makes seven different herbicide groups which waterhemp has developed resistance. As it often is when we talk about resistant weeds, this is most likely caused by repeated use of Group 15 herbicides.

While Group 15 herbicides do control many other weeds and will continue to provide waterhemp control on many farms in the near future, it’s important that we adopt multiple modes of action to achieve acceptable weed control.

We conducted an FBN member poll on March 6, 2018, in which members responded and told us which pests, diseases or weeds they were most concerned about for the growing season. Here's where U.S. farmers were most concerned about waterhemp.  

What can you do about control?

With no new herbicide groups in the near future, we must protect against resistance any way we can.

Choose herbicides that provide residual control for target weeds and should never use less than the full recommended rate. Overlapping an additional residual herbicide as a post-application helps ensure the best weed control of most targeted weeds. And remember that once crop canopy is reached, additional weeds will not be able to emerge.

Some cultural practices that can help with weed control include:

  • Row spacing

  • Crop populations

  • Cover crops

  • Tillage practices

  • Variety selection

  • Adjusted planting dates

We can’t afford to lose any more herbicide groups to resistance issues.


http://www.weedscience.org/details/Case.aspx?ResistID=18152 https://web.extension.illinois.edu/dmp/extconn/i7462_64.html


It is your responsibility to confirm prior to purchase and use that a product is labeled for your specific purposes, including, but not limited to, your target crop or pest and its compatibility with other products in a tank mix. This information should not be used as a replacement for consulting the applicable product label. Please consult the label for the most complete and up-to-date information about any referenced product. Readers must have a valid applicator license to use restricted use pesticides. Please consult your state department of agriculture for complete rules and regulations on the use of restricted use pesticides, as some products require specific record-keeping requirements.

Dual II Magnum® is a trademark of a Syngenta Group Company. Harness®, Degree® and Warrant® are registered trademarks of Monsanto Technology LLC. Surpass® is a registered trademark of Dow AgroSciences LLC. Outlook® and Zidua® are registered trademarks of BASF.

LeRoy Toohey

03 Mar 2019