Use Multiple Tools To Combat Weed Resistance

June 02, 2022

Resistant weeds continue to challenge farm yields across the country. As growers search for the best way to battle tough weeds, research scientist Dr. Lorianne Fought, Wilbur-Ellis Agribusiness, recommended a focused, integrated approach.

“To manage any pest and avoid resistance, use all your different tools to achieve as much balance as possible,” Dr. Fought explained. “Working within a threshold that is suitable for your crops will drive the economics of it.”

She recommended creating a weed management plan that helps growers stay focused and on top of weed outbreaks. “Don’t let weeds get ahead of you,” she said. “Once a field is clean, it takes less effort—and less money—to keep it clean, rather than having to do a big field clean-up.”

To get started, Dr. Fought suggested a four-step approach.

Step #1: Know what you have

Before using any weed management tools, growers need to identify which weeds they have and then consult their local university to determine if any of these weeds are resistant to any modes of action (MOAs).

Dr. Fought noted that currently about 260 species of weeds have shown some level of resistance to 23 out of the 26 MOAs in the herbicides being used.

Step #2: Understand your weed threshold

She recommended that growers determine how many weeds can be tolerated in the field before crop performance is affected. When this threshold is exceeded, that’s the point you need to intervene with weed control.

“Whether it’s bringing the weed level down to a tolerable level or supplementing the soil and nutritional programs to a level suitable for the crop, it’s all about understanding your threshold level,” she noted.

Step #3: Consider physical control

If Mother Nature cooperates, Dr. Fought suggested that growers could do a light discing preplant to remove early weed populations.

“Keep the discing light to avoid disturbing the soil biology underground as little as possible,” she noted.

Step #4: Manage chemical control wisely

When herbicide control is used, it’s important to employ strategies that provide both excellent weed control and prevent weed resistance development. To do this, Dr. Fought offered the following tips:

  • Understand how resistance develops. Every time you expose a weed population to a herbicide, you run the risk of developing a weed mutation resistant to that herbicide. By stopping seed set within the weed progeny, both future weed production and resistance are prevented.

  • Start clean, stay clean. Because it’s easier to control weeds before emergence, plan to start with a preemergent herbicide application followed by post-emergent treatments with different MOAs as needed.
  • Rotate MOAs. By rotating herbicides with different MOAs, escaped weeds that have developed resistance to the first herbicide’s MOA will be exposed to a different, effective, MOA. As long as you keep exposing weed progenies to different MOAs, you can still achieve control. When a broader population of weeds is present, use a tank mix with at least two different MOAs to achieve control.
  • Treat small weeds at full herbicide rates. To avoid weed seed set, apply herbicides when weeds are small and actively growing. This promotes better herbicide uptake for optimum control. Using the full recommended application rate combined with optimum spray coverage is also key in gaining maximum control and avoiding resistance development. Adjuvants that enhance spray coverage and herbicide uptake are also recommended.

Your local university is a good resource for effective weed control options and resistance management in your area. Contact your local Wilbur-Ellis representative for more information.