Weed management

Editor’s note: The following was written by Meaghan Anderson, Iowa State University Extension field agronomist in Central Iowa, Bob Hartzler, professor of agronomy, and Prashant Jha, associate professor, for the university’s Integrated Crop Management news website.


The plentiful rain in September has provided ideal conditions for establishment of winter annual weeds, thus many no-till fields will have dense stands of these weeds going into winter.

Horseweed seedlings

Horseweed seedlings on Sept. 26 near Ames, Iowa.

Recent wet springs have complicated getting spring burndown applications made in a timely manner. Fall burndown applications are an option that may be beneficial in fields with a history of problems with winter annuals (e.g. horseweed/marestail, field pennycress, henbit).

The advantages of fall applications include more consistent control because winter annuals are smaller and there will be less weed biomass next spring that may interfere with planter operations.

Consider the following before choosing this management option:

Scout fields following harvest to determine whether winter annuals are present and exposed through residue cover. Some winter annual populations may emerge in both the fall and spring, making effective control with a single herbicide treatment difficult.

Follow herbicide label suggestions for carrier type, carrier volume, nozzle type and environmental considerations. Treatments made on sunny days with warm daytime (more than 55 degrees) and nighttime (more than 40 degrees) temperatures will generally be more successful than those made in cooler conditions.

Winter annuals do not die after a hard freeze, so treatments will still be effective if milder conditions return.

When selecting burndown treatments, consider the likelihood of resistant horseweed biotypes in the field. HG 91 (glyphosate) and HG 2 (ALS) resistant populations are widespread across the state.

Including 0.5 lbs. ae 2,4-D LVE or 0.25 lb. ae dicamba to glyphosate will increase the consistency of horseweed control, even in fields without glyphosate resistance.

The addition of a residual herbicide in fall applications is not recommended due to the lack of consistent benefit and added expense. Residual herbicides are better left for spring herbicide applications, closer to the timeframe when most weed species are germinating.

Not all no-till fields require fall applications to control winter annuals. Situations that favor this tactic include:

  • history of high winter annual pressure,
  • presence of high weed densities at harvest,
  • presence of resistant biotypes that limit herbicide options in the spring, and
  • factors that prevent timely applications in the spring while weeds are small (poorly drained fields, sprayer availability, etc.).

Effective control of winter annuals prior to planting is an important first step for weed management in no-till, and in some fields starting clean in 2020 will benefit from some effort this fall.

While fall-applied herbicides will reduce the amount of vegetation present next spring, they rarely eliminate the need for controlling established vegetation at planting.

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