There are many more options to control horseweed (marestail) in soybean and cotton than what was available just a few years ago. The biggest new option for many acres of Xtend cotton or soybean has been the use of Engenia or XtendiMax closer to planting than the standard 21 days before planting use of Clarity.
Sharpen or the premix of Sharpen and Outlook (Verdict) has been the most utilized horseweed burndown option in recent years. In burndown scenarios, 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.
New for horseweed control in 2018 is Elevore. Elevore’s new active ingredient is halauxifen-methyl. Its best fit is as a burndown prior to corn, cotton, and soybeans. Elevore is in the same herbicide family (Group 4) as 2,4-D and dicamba. The plant back is 14 days prior to planting soybean and corn and 30 days prior to planting cotton. Elevore can be applied at 1.0 fl. oz./A with no more than two pre-plant applications per year. In our trials, Elevore at 1.0 fl. oz./A has been very effective on horseweed up to 8 inches tall.
Though these herbicides mentioned above can be effective burndown options for horseweed, we highly recommend tank-mixes for resistance management purposes. Tank-mixes of dicamba + Sharpen or Verdict, Sharpen + Elevore, or Elevore + FirstRate or Surveil have provided greater than 95% control of 8” horseweed in our research. These tank-mixes can remove the regrowth potential we oftentimes see in burndown applications on horseweed (especially fields we can’t get into on time). These tank-mixes should also include herbicides with grass activity, i.e. Roundup Powermax or Select Max (30 day plant back to corn for any product that contains clethodim).