Mark VanGessel, Extension Weed Specialist; mjv@udel.edu
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 quite effective on horseweed (be sure to keep your rates up and keep spray volume up to ensure good coverage).
For non-Liberty Link soybeans the options are very limited. I have had very inconsistent results trying to control horseweed with ALS products (such as FirstRate, Classic or Synchrony). Part of this is due to ALS resistant horseweed populations scattered around the region; and partly due to application to larger weeds or weeds “burnt off” and starting to recover. If you want to try either FirstRate or Synchrony on STS soybeans, use the highest rate allowed and full-adjuvant systems (refer to the labels). Horseweed plants are generally not very tolerant of shade and most soybeans will 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. It is always best to treat the horseweed soon after they start regrowing from the burndown application.