By William Curran, Weed Scientist; Dwight Lingenfelter, Program Development Specialist

There’s been a lot of discussion about horseweed/marestail management in soybeans in recent years. Historically, glyphosate-resistant marestail was limited to southeastern Pennsylvania down into the Delmarva and west in Ohio, but now it is much more common in central Pennsylvania and has appeared in the western part of the state as well. 

This weed is mostly a problem in no-till and more of a problem in soybean than corn. It’s also a common site along the margins of some fields where herbicide programs are usually more hit or miss.

Ohio and Indiana farmers are also battling marestail and it’s not uncommon for them to have both glyphosate and Group 2 or ALS-inhibitor resistant biotypes. The use of residual herbicides to control escapes has become more common in some areas and particularly in Ohio and Indiana where 6-8 weeks of residual control may be necessary to manage spring emerging marestail. 

Here in the Mid-Atlantic, the need for residual herbicides for marestail control is less clear as it appears that most of our marestail emerges in the fall and early spring prior to burndown herbicide applications. However, be on the lookout as you scout fields this spring and early summer. Residual soybean herbicides can help manage not only late-emerging marestail, but also all those other summer annual broadleaves that are problematic.

Here are the general guidelines that we continue to recommend for marestail management in our area:

  • Start clean and control marestail prior to soybean emergence.
  • Control marestail when it’s small, while it’s still in the rosette stage and prior to bolting.
  • Use 2,4-D and/or Shapen, Verdict, Optill (saflufenacil) in the burndown program to control emerged plants. Saflufenacil use on coarse soils with 2% or less organic matter is more restrictive.
  • Applications in April should include a residual herbicide to control later emerging plants.
  • Plant a Liberty-Link soybean variety if possible and use an in-crop, post herbicide if necessary. Bayer introduced Credenz soybean this year, which is the first commercially available Roundup Ready-Liberty Link stack varieties. This should help provide some flexibility with post-emergence herbicide selection. 
  • In our trial last year at Landisville, a two-pass program of glyphosate only provided 67% control. A two-pass program that included FirstRate with glyphosate post provided 84% control.

There are recommendations/restrictions for tank-mixing saflufenacil with other Group 14 or PPO herbicides due to the concern for soybean injury. According to the BASF labels, you must wait at least 14 days if you mix 1 fluid ounce of Sharpen (or 5 fluid ounces of Verdict) with other Group 14 herbicides (more than 1 fluid ounce of Sharpen requires 30 days).

Soil-applied Group 14 herbicides include the following:

  • Flumioxazin products — Valor, Valor XLT, Envive, Trivence, Afforia, Fierce, Surveil, Gangster, Rowel, Rowel FX (and other labeled formulations)
  • Sulfentrazone products — Authority products, Sonic, BroadAxe XC
  • Fomesafen products — Prefix, Reflex, Warrant Ultra. Syngenta does have a Section 2(ee) label for Prefix tank-mixed with 1 fluid ounce of Sharpen as a pre-plant application in soybean. We are not sure what Syngenta’s definition of pre-plant is, but this product is being promoted locally for application 2-3 days before soybean emergence.
  • Other residual non-Group 14 broadleaf herbicides such as metribuzin, Canopy, FirstRate, etc., can be tank-mixed with saflufenacil and applied pre-emergence.