For no-tillers, there’s no shortage of herbicide-resistant weeds to worry about, and for Oklahoma State University Extension’s integrated pest management program for cotton, glyphosate-resistant horseweed, or marestail, is firmly in the bulls-eye.

Although horseweed can be controlled well with traditional auxin herbicides like

2,4-D or dicamba, the university has been exploring additional options in hopes of finding effective combinations that could increase the sustainability of auxin products for the future.

Researchers last year evaluated DuPont’s new herbicide, Afforia, for controlling horseweed and other winter weeds prior to planting cotton. Combinations of Afforia with 2,4-D and glyphosate effectively controlled horseweed ahead of cotton planting, the university says.

Afforia is a combination of flumioxazin, thifensulfuron methyl and tribenuron methyl — active ingredients formerly associated with Valor, Harmony and Express, respectively. All of the products provide broadleaf weed control with limited residual carryover to cotton of 14-30 days.

At the Southwest Research and Extension Center in Altus, Okla., Afforia was applied in early April last year to control horseweed and other winter weeds, and compared to several other treatments considered standard options for this timing, such as 2,4-D or paraquat.

Treatments of Afforia, 2,4-D and glyphosate effectively controlled henbit, redstem filaree and rescuegrass, which was similar to all other treatments, the university says. But horseweed was controlled more effectively when Gramoxone was included in the application, compared to all other treatments. The reason may be that the horseweed, at the time of application, had already bolted and was 3-6 inches tall.

For no-till cotton growers, Oklahoma State issued these recommendations this year to control marestail — the first of which is, of course, to start planting with clean fields and keep them clean. Here are a few other tips:

• When using glyphosate, tankmix 1 pound active ingredient per acre of 2,4-D or 0.25 pounds active ingredient of dicamba.
• Spray marestail when weeds are small — rosettes are easiest to control.
• Don’t forget about labeled plant-back intervals: 30 days after 2,4-D, and 21 days after a 1-inch rainfall following dicamba.

Is herbicide-resistant marestail a problem in your no-till cotton? What is your plan for controlling this weed this season? Share your experiences and thoughts in the comments below.