| Horseweed (Conyza canadensis), a member of the Asteraceae family, is an annual plant that can behave as either a winter annual or summer annual. It predominantly self-pollinates and can produce in excess of 1,000,000 wind-blown seeds. While it is commonly thought to be an early succesionary plant in roadsides, along railroad tracks, and in fallow areas, it has become problematic in no-till soybean production. It is most notorious because of its ability to rapidly evolve resistances to various chemical herbicides. It was the first reported broadleaf weed with resistance to glyphosate in the U.S. and has since been reported in numerous states.;Horseweed populations in Indiana have been shown to have evolved resistance to glyphosate and ALS-inhibitors. While the mechanism of resistance to glyphosate is unknown, resistance to ALS-inhibiting herbicides has been shown to be due to multiple different mutations in the ALS enzyme. Consequently, the responses of select populations with different mutations respond differently to different families of ALS-inhibiting herbicides. Horseweed populations with multiple-resistance to glyphosate and ALS-inhibiting herbicides have been found in Indiana, and they too exhibit unique responses to tank-mixtures of the two herbicides.;Growth regulators are often recommended as a means for chemical management of horseweed populations with resistance to other modes of action. Indiana horseweed populations were found to exhibit differences in response to some formulations of 2,4-D and dicamba, creating some concern for the future evolution of resistance to this particular mode of action as well, despite the fact that known resistance to growth regulators does not yet exist in horseweed. Because of this, it is important to use sound stewardship practices to prolong the efficacy of growth regulator herbicides. |
