Historically glyphosate resistant horseweed was limited to the southeastern Pennsylvania down into the Delmarva and West in Ohio, but now it is much more common in central PA and has appeared in western PA as well.
There has been a lot of discussion about horseweed/marestail management in soybeans in recent years. Historically glyphosate resistant horseweed was limited to the southeastern Pennsylvania down into the Delmarva and West in Ohio, but now it is much more common in central PA and has appeared in western PA as well. This weed is mostly a problem in no-till and more of a problem in soybean than corn. It is 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 horseweed and it is 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 for common in some areas and particularly in Ohio and Indiana where 6 to 8 weeks of residual control may be necessary to manage spring emerging horseweed. Here in the Mid-Atlantic, the need for residual herbicides for horseweed control is less clear as it appears that most of our horseweed 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 and residual soybean herbicides can help manage not only late emerging horseweed, but also all those other summer annual broadleaves that are problematic.
Here are the general guidelines that we continue to recommend for horseweed/maretail management in our area:
- Start clean and control horseweed prior to soybean emergence.
- Control horseweed when it is small, while it is 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 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 fl. oz. of Sharpen (or 5 fl. oz. Verdict) with other Group 14 herbicides (More than 1 fl. oz. Sharpen requires 30 days).
Soil applied Group 14 herbicide 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 fl. oz. Sharpen as a preplant application in soybean. We are not sure what Syngenta’s definition of Preplant is, but this product is being promoted locally for application 2 to 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.