Baseline Sensitivity of Nebraska Waterhemp and Palmer Amaranth to Dicamba and 2,4‐D

Abstract

The development of dicamba (3,6‐dichloro‐2‐methoxybenzoic acid)‐ and 2,4‐D [(2,4‐dichlorophenoxy) acetic acid]‐resistant soybean [Glycine max (L.) Merr.] will increase the frequency of applying these herbicides to control broadleaf weeds in soybean production systems. We evaluated the sensitivity of randomly selected waterhemp [Amaranthus tuberculatus (Moq.) Sauer var. rudis (Sauer) Costea and Tardif] and Palmer amaranth (Amaranthuspalmeri S. Wats) populations to a single dose of dicamba and 2,4‐D. Selected populations were then used in dicamba and 2,4‐D dose‐response experiments to evaluate: (i) visual injury at 28 d after treatment; and (ii) plant survival, biomass accumulation, and flowering at 84 DAT. Across selected populations, the estimated dicamba dose for 50% visual injury (I50) was below the recommended field rate (560 g ha−1) and ranged between 59 and 117 g ha−1 for waterhemp and 43 and 81 g ha−1 for Palmer amaranth populations. The estimated 2,4‐D dose for I50 was also below the recommended field rate (800 g ha−1) and ranged between 127 and 242 g ha−1 for waterhemp and 127 and 196 g ha−1 for Palmer amaranth. Waterhemp plants flowered with up to 560 g ha−1 dicamba and 2240 g ha−1 2,4‐D, while Palmer amaranth plants flowered with up to 280 g ha−1 dicamba, and 1120 g ha−1 2,4‐D. Individual plants will likely survive and flower following recommended field rates of dicamba (waterhemp) and 2,4‐D (both species). Close monitoring of weed populations are needed for the stewardship of available weed management technologies.Core Ideas We conducted a state‐wide baseline study of auxinic herbicide sensitivity in randomly selected waterhemp and Palmer amaranth populations. Individual plants of both species will likely survive and flower after field rates of dicamba and/or 2,4‐D. Close monitoring of weed populations and continuous update of best management practices are needed for the stewardship of available weed management technologies. This study sets a benchmark to detect shifting in sensitivities to dicamba or 2,4‐D after resistant crops are released.