Mark VanGessel, Extension Weed Specialist; firstname.lastname@example.org
Options for controlling horseweed resistant to glyphosate after the soybeans have emerged are very limited. Liberty Link soybeans are an exception, because Liberty 280 is fairly effective on horseweed (be sure to keep your rates up).
For non-Liberty Link soybeans the options are very limited. Liberty Link soybeans can be treated with Liberty (or Ignite) for fair control of horseweed. For non-Liberty Link soybeans, FirstRate or Classic are effective on small, newly emerged seedlings. However, neither will consistently kill large horseweed plants, nor plants that were “burned off” and are recovering (FirstRate is better option than Classic after the soybeans have emerged). These herbicides may provide some suppression, but results have been quite erratic the past few years. Horseweed plants are generally not very tolerant of shade and most soybeans will begin to canopy over the horseweed and out-compete them. Additional glyphosate applications will provide some suppression of horseweed and sometimes the soybeans have a chance to outcompete them.