Sensitivity and resistance survey of common lambsquarters (Chenopodium album) populations to bentazon across the United States

Bentazon, a photosystem II–inhibiting postemergence herbicide, has been used in corn (Zea mays L.), soybean, wheat (Triticum aestivum L.), and vegetables to manage common lambsquarters (Chenopodium album L.), although growers have reported reduced efficacy across the country. The aim of this study was to describe the sensitivity response of C. album to bentazon and identify whether reported escapes could be considered to be cases of herbicide resistance evolution. We evaluated C. album populations collected from lima and snap bean (Phaseolus vulgaris L.) fields across Delaware, Illinois, Minnesota, New York, and Oregon. Dose–response experiments with 25 populations were conducted to create a reference response to bentazon, using rates that ranged from 0 to 8,406 g ai ha⁻¹. Injury ratings and biomass were assessed at 28 d after herbicide application, and the herbicide rates required to reduce growth by 50% (I₅₀ or GR₅₀) and 80% (I₈₀ or GR₈₀) were calculated. Results indicated C. album responses to bentazon varied within and across states. Across all populations studied, the GR₅₀ for biomass reduction ranged from 159 to 816 g ha⁻¹, and GR₈₀ from 230 to 1,944 g ha⁻¹ with populations from Oregon exhibiting the highest average GR₅₀, followed by those from New York, the Northcentral states, and Delaware. Based on our criteria that the injury rating- or biomass-based resistance index (ratio between I₅₀ or GR₅₀ of the suspected resistant and a local selected susceptible population) had to be at least 2, and the I₈₀ or GR₈₀ should be greater than the labeled field rate, one population from New York (NY6) and one from Oregon (OR29) were considered to be resistant. This research underscores the wide variation in C. album response to bentazon across the United States and the importance of herbicide resistance diagnostic strategies that account for local population variation, and highlights the increasing challenge of C. album management in specialty crops.