Horseweed/Marestail in winter wheat can create problems for double-crop soybeans (image by Chris Drake, Virginia Cooperative Extension).
Controlling weeds before soybean emergence is a critical management goal for double crop soybeans. Horseweed or marestail control can be challenging in double crop soybeans for several reasons. Horseweed plants can be large and/or they have been cutoff with the combine, making them more challenging to control. Dr. Mark VanGessel from the University of Delaware does not recommend 2,4-D because of off-target movement at this time of year and he does not think it is very effective on large or damaged plants. And of course, the 7-day waiting period following 1 pt is also not attractive in double crop. Sharpen is an option for horseweed control. It must be applied before soybean emergence and is applied at 1 oz/acre. Liberty 280 may be applied at up to 36 fl oz as a burndown treatment prior to soybean emergence. Dr. Mark Loux at Ohio State suggests the following three burndown options:
One of the best options is to plant LibertyLink soybeans and apply Liberty POST to help control plants that survive the burndown. Also consider products that contain chlorimuron or cloransulam (Canopy, Envive, Valor XLT, Authority First, and Sonic) to help suppress horseweed and provide residual control of other annual broadleaf weeds. However, keep mind that some horseweed in the region is also resistant to the Group 2 or ALS inhibitor herbicides.