Mark VanGessel, Extension Weed Specialist; mjv@udel.edu
On June 8, EPA issued a ruling to allow use of existing stocks of dicamba on soybeans. Read DE Department of Agriculture press release here https://news.delaware.gov/2020/06/09/delaware-growers-allowed-to-use-up-existing-dicamba-stock/.
Dicamba that was in the possession of commercial and private applicators as of June 3 can be sprayed through July 31. All precautions and restrictions on sensitive crops, temperature, wind speed and direction, and application equipment are still in place. As mid-June approaches, temperatures begin to increase, and this can increase the risk of off-target movement. Most of these products recommend additional precautions when temperatures exceed 91°F to compensate for evaporation.
If you had intended to use one of the formulations of dicamba impacted by this ruling, but did not have possession of the product, there are some available options.
Palmer Amaranth: Fomesafen-products (i.e. Reflex, Flexstar GT), acifluorfen (i.e. Ultra Blazer), or lactofen (Cobra) will provide control of emerged plants. Fomesafen will also provide residual control whereas acifluorfen or lactofen will not. Be aware the use of a residual herbicide with acifluorfen and lactofen can increase the severity of leaf burn; however, without the residual herbicide there is a good chance of later emerging plants.
Common Ragweed: It depends on which type of resistance you have in your field. From our testing, the most wide-spread resistance is glyphosate plus ALS-resistance, there are some fields resistant to glyphosate plus ALS-herbicides plus PPO-resistance. So if you were intending to use dicamba because of glyphosate-resistant common ragweed, then fomesafen, acifluorfen, or lactofen are the best options. Even if you know you have PPO-resistance, these products will burn off most of the common ragweed leaves and so you can set back the weeds and maybe the soybean canopy will help with outcompeting a number of these plants.
Horseweed: Most of our horseweed-plants are also resistant to FirstRate so chances are this is not a good choice. None of the other herbicides will control horseweed either. My suggestion is use glyphosate as part of your postemergence spray; glyphosate will stunt the horseweed and if there are good growing conditions the soybean canopy will help suppress the horseweed.
Regardless of the postemergence herbicide you intend to use, be sure to spray while weeds are small (3 inches or less) to achieve the highest level of control possible.