Fall applied herbicides

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Fall Applied Herbicide Strategies

Steve Doench, Head of Sustainable Agronomy
Steve Doench, Head of Sustainable Agronomy

This season crops are progressing well, and the harvest timeframe looks to be on time to slightly ahead of schedule.  With the outlook looking good for timely harvest, the option of applying fall herbicides looks to be more available than in years with delayed crop maturity and a later harvest window.

Fall herbicide applications are a useful strategy for controlling difficult weeds such as winter annuals like marestail as well as biennial and perennial weeds.  In the fall, these types of weeds are easier to control.  Fall herbicide applications typically involve using a tank mix of a post-emergence herbicide like glyphosate, dicamba, or 2,4-D with a residual herbicide that is compatible with the crop to be planted in the spring.

Canadese fijnstraal plant Conyza canadensis

The benefit of fall herbicide applications is especially useful for no-till systems that rely on good pre-season weed control.  After successful fall herbicide applications, growers should expect clean fields until planting is underway, allowing for better crop establishment in the spring.

Fallow ground weed control

Is there a best time to do fallow ground weed control?

Fields that have been left fallow for a season or longer most likely have a significant population of weeds growing and have been building their weed seed bank. Although tillage is an option to control the weeds, it might spread weeds that grow through vegetative propagation. It also will not prevent weeds from germinating in the spring. Tillage might also promote undesirable erosion.

To be adequately controlled by herbicide, weeds need to be actively growing. Therefore, the end of summer and early fall can be a good time to start preparing the fields for next season growing. However, seeds will continue to germinate before and during the growing season, so multiple applications of pesticides might be necessary to control the germination of seeds that are in the weed seed bank over time.

Selecting the right herbicide is important. A contact herbicide typically works better on smaller weeds than larger ones. Systemic pesticides move through the plant and might be better for larger weeds present later in the season. Contact herbicides often do not have any soil residual activity. They can be used shortly before the planting of the crop, while systemic pesticides often require an application at least ninety days before planting crops.

Things to consider when selecting the right herbicide for fallow grounds are:

  • The type of weed
  • The timing of the application
  • Size of weeds
  • Mode of action of the herbicide

General fall herbicide application tips

Apply fall herbicides in mid-late fall before the ground is frozen and when weeds are still growing and not already frosted off.  When a residual herbicide is part of the tank mix, waiting until soils are below 50 degrees will allow for their persistence and extended control into early spring.

  • Use higher use rates than for spring burndown applications.
  • Consider using multiple modes of action when possible.
  • Only apply residual herbicides that are compatible with the crops to be planted in the spring.
Apply fall herbicides in mid-late fall before the ground is frozen and when weeds are still growing and not already frosted off.

Always follow label instructions for rate and recommended adjuvants to ensure proper application to prevent herbicide resistance. Fall weed management strategies should always include label-recommended adjuvants. Effective applications include the combination of the right active ingredient, the application right timing, and effective delivery of the AI to the target weed. Adjuvants that improve spreading, deposition, or penetration of the active ingredient through waxy leaf cuticles can help ensure your fall herbicide application is effective.

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