Plentiful rain in September has provided ideal conditions for establishment of winter annual weeds. Many no-till fields will have dense stands of these weeds going into winter. The wetter springs Iowa has encountered in recent years complicate getting spring burndown herbicide applications made in a timely matter.
How about applying a burndown herbicide treatment in the fall instead of spring? To answer that question, Iowa State University Extension weed management specialist Bob Hartzler, with the assistance of ISU weed scientist Prashant Jha and field agronomist Meaghan Anderson, offers the following observations and recommendations.
Fall burndown herbicide applications are an option that may be beneficial in fields with a history of problems with winter annual weeds such as horseweed or marestail, field pennycress, and henbit. The advantages of fall applications include more consistent control since winter annuals are smaller and less weed biomass next spring to interfere with planter operations.
3 key recommendations
Consider these guidelines before choosing fall herbicide treatment as a weed management option:
1. Scout fields following harvest. Determine whether winter annual weeds are present and are exposed through the crop residue cover. Some winter annual populations may emerge in both the fall and spring, making effective control with a single herbicide treatment difficult.
2. Follow herbicide label. Heed suggestions for carrier type, carrier volume, nozzle type and environmental considerations. Treatments made on sunny days with warm daytime temperature (over 55 degrees F) and nighttime temps (over 40 degrees) 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.
3. Watch for resistant horseweed. When selecting burndown treatments, consider the likelihood of resistant horseweed biotypes in the field. Weed populations that are resistant to Herbicide Group 9 products (glyphosate) and HG 2 products (ALS herbicides) are widespread across the state. Including 0.5-pound acid equivalent 2,4-D (low-volatile ester) or 0.25-pound a.e. dicamba — added to the glyphosate — will increase the consistency of horseweed control, even in fields without glyphosate-resistant weeds.
Adding a residual herbicide to the fall burndown treatment is not recommended due to the lack of consistent benefit and because of the added expense. Residual herbicides are better left for spring herbicide applications, closer to the time frame when most weed species are germinating.
Is fall-applied herbicide needed?
Not all no-till fields require fall applications to control winter annual weeds. Situations that favor this tactic include:
- fields with a history of high winter annual weed pressure
- presence of high weed densities at harvest
- presence of resistant weed biotypes that limit herbicide options in the spring
- 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. In some fields, starting clean in 2020 will benefit from some effort this fall to control the weeds. While fall-applied herbicides will reduce the amount of vegetation present next spring, they rarely eliminate the need for controlling established vegetation at planting time.