There are reports and suspicions of additional fields infested with glyphosate-resistant horseweed in Delaware, Maryland and New Jersey. Horseweed is a winter annual weed.
Emergence begins in the fall and continues into the early summer.
Resistant-horseweed can tolerate glyphosate 'treatments at all stages, even while the plant is in the rosette stage. Glyphosate is the active ingredient in Roundup, Touchdown, Glyphos, GlyphoMax, and numerous other brand names. If resistance is suspected in one of your fields, some questions should be asked to help determine if herbicide resistance is the reason for lack of control. In trying to determine if herbicide resistant weeds are present, first eliminate other causes of herbicide failure. Resistance develops by selecting resistant-biotypes through repeated use of the same herbicide mode of action.