Mark VanGessel, Extension Weed Specialist; firstname.lastname@example.org
We confirmed a population of horseweed (or marestail) in Delaware that is resistant to glyphosate and ALS-inhibiting herbicide. This field had been treated with Finesse and Canopy EX and glyphosate and the horseweed was not controlled. We collected seed from this field and tested it in the greenhouse. This is the first confirmation of multiple resistance for horseweed in the Mid-Atlantic states. Glyphosate will not control this biotype, but neither will chlorimuron or FirstRate. Chlorimuron is the active ingredient in Classic, Canopy SG, Canopy EX, Synchrony, Envive, and Valor XLT. Controlling these populations will require 2,4-D (at 1 qt/A for consistent control), Ignite 280, or Kixor (but Kixor needs to be applied at least 30 days prior to planting in coarse-textured soils). The most cost-effective approach will be use of 2,4-D applied at 1 qt/A at least 30 days prior to planting. Delaying the 2,4-D applications until closer to planting will result in lower rates of 2,4-D being applied, less effective horseweed control, and more situations that are not appropriate for 2,4-D because of emergence of nearby crops and vegetables.
NOTE: There are many circumstances where 2,4-D is not an option due to susceptible plants or greenhouses in the area. Be sure to know the surrounding area before you treat a field with 2,4-D to be sure it is an appropriate treatment for the area.