A population of common waterhemp in northeast Kansas was confirmed resistant to protoporphyrinogen oxidase (protox)-inhibiting herbicides in 2001. In 2002, seeds were collected from 28 sites in a 16-km radius surrounding the site where resistance was confirmed to determine the extent of protox resistance in common waterhemp populations throughout the area. In addition, common waterhemp response to acetolactate synthase (ALS)-inhibiting herbicides and glyphosate was determined. At least one common waterhemp plant among the 48 plants tested from each of 10 sites was acifluorfen-resistant. These sites were randomly scattered throughout the sampling area, and resistance may have resulted from seed or pollen movement or independent development. Acifluorfen-resistant common waterhemp plants were initially injured by acifluorfen, but plants began recovering from injury within 14 days after treatment (DAT). All sites contained at least two common waterhemp plants with imazethapyr resistance, whereas plants from all sites were susceptible to glyphosate. Because acifluorfen- and imazethapyr-resistant common waterhemp populations are found in northeastern Kansas, protox-inhibiting and ALS-inhibiting herbicides may not provide common waterhemp control.

Resistance to protoporphyrinogen oxidase (protox)-inhibiting herbicides was identified in a population of common waterhemp that had been treated with acifluorfen for several years. The protox-resistant biotype of common waterhemp was approximately 34, 82, 8, and 4 times more resistant than a susceptible common waterhemp biotype to acifluorfen, lactofen, fomesafen, and sulfentrazone, respectively. The resistant biotype also showed a high level of resistance to acetolactate synthase–inhibiting herbicides thifensulfuron and imazethapyr but not to glyphosate or paraquat. An organophosphate insecticide was applied with acifluorfen or lactofen to determine if metabolism could be the mechanism of resistance. No differences were observed between resistant plants treated with an organophosphate plus a protox-inhibiting herbicide and plants treated with a protox-inhibiting herbicide alone.