Before you plant soybeans is the time to consider horseweed/marestail management. 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.
Much of the horseweed in the region is glyphosate resistant and pockets of Group 2 or ALS-inhibitor resistant biotypes also exist. The use of residual herbicides to control escapes is becoming more common where 6 to 8 weeks of residual control may be necessary to manage spring emerging horseweed.
Herbicide Resistance Info
Be sure you are scouting your no-till soybean fields this spring and early summer and selecting appropriate control measures. Rescue options after soybeans emerge are fewer and require upfront preparation using either Liberty Link or Xtend soybean varieties.
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, or Zidua Pro (saflufenacil containing products) in the burndown program to control emerged plants. Saflufenacil use on coarse soils with 2% or less organic matter is more restrictive.
If you are planting Xtend soybean, the dicamba based products Engenia, FeXapan plus Vapor Grip Technology, and Xtendimax with Vapor Grip Technology can be used preplant or preemergence for horseweed/marestail control. Click here for the April 12 FCN article that goes into more detail on use guidelines.
Burndown applications in April and early May 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.
In our trials over the last few years at Landisville, a two-pass program of glyphosate provided about 65% maretail control. Glyphosate mixtures with 2,4-D LVE, saflufenacil, or cloransulam (FirstRate, Authority First, or Sonic) can greatly improve control.
The new dicamba products applied preplant have typically provided 90%+ control of marestail in our trials. Adding metribuzin or one of several Group 14 herbicides (flumixazin, fomesafen, or sulfentrazone) will improve residual control. Remember that there are restrictions for tank-mixing saflufenacil with other Group 14 herbicides due to the concern for soybean injury. Check the most current label for what is allowed.