A waterhemp population from a native-grass seed production field in Nebraska was no longer effectively controlled by 2,4-D. Seed was collected from the site, and dose-response studies were conducted to determine if this population was herbicide resistant. In the greenhouse, plants from the putative resistant and a susceptible waterhemp population were treated with 0, 18, 35, 70, 140, 280, 560, 1,120, or 2,240 g ae ha⁻¹ 2,4-D. Visual injury estimates (I) were made 28 d after treatment (DAT), and plants were harvested and dry weights (GR) measured. The putative resistant population was approximately 10-fold more resistant to 2,4-D (R∶S ratio) than the susceptible population based on both I₅₀ (50% visual injury) and GR₅₀ (50% reduction in dry weight) values. The R∶S ratio increased to 19 and 111 as the data were extrapolated to I₉₀ and GR₉₀ estimates, respectively. GR₅₀ doses of 995 g ha⁻¹ for the resistant and 109 g ha⁻¹ for the susceptible populations were estimated. A field dose-response study was conducted at the suspected resistant site with 2,4-D doses of 0, 140, 280, 560, 1,120, 2,240, 4,480, 8,960, 17,920, and 35,840 g ha⁻¹. At 28 DAT, visual injury estimates were 44% in plots treated with 35,840 g ha⁻¹. Some plants treated with the highest rate recovered and produced seed. Plants from the resistant and susceptible populations were also treated with 0, 9, 18, 35, 70, 140, 280, 560, or 1,120 g ae ha⁻¹ dicamba in greenhouse bioassays. The 2,4-D resistant population was threefold less sensitive to dicamba based on I₅₀ estimates but less than twofold less sensitive based on GR₅₀ estimates. The synthetic auxins are the sixth mechanism-of-action herbicide group to which waterhemp has evolved resistance.

Nomenclature: 2,4-D; dicamba; waterhemp, Amaranthus tuberculatus (Moq.) Sauer var. rudis (Sauer) Costea and Tardif AMATU.