A population of waterhemp was identified in Adams County, Illinois, that survived treatment of several acetolactate synthase (ALS) inhibitors and a postemergence (POST) application of lactofen, a protoporphyrinogen oxidase (PPO)–inhibiting herbicide. Greenhouse studies were conducted to quantify the responses of this waterhemp population, designated ACR, to multiple PPO inhibitors and various other herbicides with different sites of action. Resistance ratios were obtained by comparing herbicide dose–response curves between the ACR population and a herbicide-susceptible waterhemp population. The ACR population was resistant to lactofen (23-fold) and to five other PPO-inhibiting herbicides (ranging from 2.2- to 6.2-fold). Furthermore, the ACR waterhemp population was 17,000-fold and 18,000-fold resistant to imazamox and thifensulfuron, respectively, two ALS-inhibiting herbicides. In response to atrazine, a Photosystem II inhibitor, the ACR population was 38-fold resistant. Plants within the ACR waterhemp population survived treatment of a herbicide mixture containing lactofen at 175 g ai ha−1, imazamox at 44 g ae ha−1, and atrazine at 1,000 g ai ha−1. Thus, individual plants—not just the population as a whole—displayed multiple herbicide resistance. The ACR population was not resistant to glyphosate or paraquat. This is the first reported weed population from the United States with resistances to herbicides inhibiting three unique sites of action. Furthermore, this research identifies a significant reduction in the number of POST herbicide options available for waterhemp control in soybean production.
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