Wallaces Farmer

Soybean Source: Evaluating herbicide rates, properly timing applications and scouting are ways to keep weeds at bay.

Meaghan Anderson

March 22, 2024

3 Min Read
waterhemp
PREVENTING A MESS: Properly evaluating herbicide rates and performing timely applications laced with cultural methods such as boosting crop competition can nix a waterhemp mess like this one. Gil Gullickson

With the 2024 growing season upon us, burndown, preplant and preemergence herbicide applications should be top-of-mind for those interested in keeping ahead of weed issues. With each new year, new weed problems arise, and old problems become harder to control. How will we beat the weeds in 2024?

In the past six months, Bayer reported the discovery of an Iowa waterhemp population resistant to dicamba, while Corteva reported a 2,4-D resistant population in the state. While neither of these discoveries is surprising, it emphasizes how quickly dicamba and 2,4-D could be lost as options for controlling waterhemp in soybeans and corn.

These discoveries are just individual examples of what occurs every time we use 2,4-D or dicamba to control waterhemp (or other species) around farm buildings, in road ditches or in crop fields. The cause of this resistance: repeatedly using the same herbicide to control a weed. In response to repeated use, we select individuals that survive those herbicides, and allow them to produce seed and become the majority genetics for the population.

Change it up

So how do we keep the weeds from beating us this year and in the future? The only way to prevent new resistances from developing in crop fields is to prevent adding weed seed to the soil. This spring, plan herbicide programs that fit the weed spectrum in individual fields, which may mean increasing rates or changing herbicide active ingredients in some fields.

Using multiple herbicide groups is important, but the next step is to evaluate each active ingredient to determine whether rates are sufficient to keep waterhemp at bay. For example, many herbicide premixes provide less than the full labeled rate of each active ingredient (see accompanying chart).

Postemergence herbicide applications have less flexibility in rate than preemergence herbicides, but the timing of these applications, carrier volume, adjuvant and nozzle choice are all critical to effectively control waterhemp.

Applications must be made to waterhemp that is less than 4 inches tall. Inclusion of a residual herbicide from herbicide Group 15 (dimethenamid, pyroxasulfone, metolachlor, etc.) is important to suppress waterhemp that may emerge after the postemergence spray.

In the seven to 10 days following postemergence application, farmers should scout fields to determine the effectiveness of that application. If waterhemp survives an application and the farmer suspects resistance is the cause, it is important to alert the herbicide product manufacturer to the issue. Farmers should make every reasonable effort to clean up escapes.

Non-chemical options

Monitoring weeds throughout the season and adjusting plans based on conditions and field-specific issues, such as different planting dates and heavier weed pressure, are key to improving weed control. In the long term, we need to determine where to best incorporate non-chemical weed management strategies. These include:

  • making crops more competitive

  • suppressing weed emergence

  • using mechanical strategies to eliminate weeds and reduce seed additions to the soil

These won’t replace herbicides, but they will make herbicides work longer into the future.

How do we stay ahead of the weeds? Using ingenuity to find success with options beyond herbicides is really the only way.

About the Author(s)

Meaghan Anderson

Meaghan Anderson is the Iowa State University Extension field agronomist for east-central Iowa. He areas of expertise include weed management, weed biology, cover crops, corn and soybean management, and Integrated Pest Management.

Subscribe to receive top agriculture news
Be informed daily with these free e-newsletters

You May Also Like