Larry Steckel, Extension Weed Specialist
Horseweed (marestail) has made a major comeback in 2025! The last time horseweed infestations were this intense in so many fields this late in the spring was in 2017. The question is why? I can point to one likely reason.
I think the biggest reason is the reduced use of dicamba in burndown applications. In an effort to do a better job controlling ryegrass many are keeping dicamba out of the tank. This has helped on ryegrass control but gives horseweed a pass. Moreover, we have seen a huge increase in the use of Valor for residual control in burndowns. In most cases the Valor was applied after horseweed had emerged. Valor will not provide any POST control of horseweed but will keep other winter annual weeds from emerging which reduces the competition between those weeds and horseweed.
Unlike a decade ago, there are several herbicides that will provide good control in horseweed in our row crops. In corn, most any premix that contains an HPPD-inhibiting herbicide (Armezon, Callisto, Laudis, Shield Ex, etc.) mixed with atrazine will control even large horseweed. In soybean or cotton, Liberty applied on a very warm day will control horseweed. Enlist One tankmixed with Liberty will control horseweed in Enlist crops and will be effective in cooler conditions that are not optimal for Liberty alone.
With all those options in crop, it is not as critical that horseweed be controlled before planting soybeans. There are exceptions in fields with intense infestations (Picture 1). In those soybean fields, burn down with Sharpen or the premix of Sharpen and Outlook (Verdict). In burndown scenarios where the horseweed is small, 1.0 fl oz/acre of Sharpen or 7.5 fl oz/acre of Verdict are the recommended rates. Once horseweed gets taller than 6” a mixture of 1.0 oz of Sharpen or 7.5 ozs of Verdict with 0.25 lbs/acre of 2,4-D or dicamba has provided much more consistent control. The Outlook component in Verdict adds residual control of annual grasses and small-seeded broadleaves. Both require MSO at 1% v/v to be added to the tank to optimize activity. These herbicides are fast-acting and can be an effective option in timely horseweed applications.
