Metabolism of 2,4‐dichlorophenoxyacetic acid contributes to resistance in a common waterhemp ( Amaranthus tuberculatus ) population

BACKGROUND Synthetic auxins such as 2,4‐dichlorophenoxyacetic acid (2,4‐D) have been widely used for selective control of broadleaf weeds since the mid‐1940s. In 2009, an Amaranthus tuberculatus (common waterhemp) population with 10‐fold resistance to 2,4‐D was found in Nebraska, USA. The 2,4‐D resistance mechanism was examined by conducting [ 14 C] 2,4‐D absorption, translocation and metabolism experiments. RESULTS No differences were found in 2,4‐D absorption or translocation between resistant and susceptible A. tuberculatus plants. Resistant plants metabolized [ 14 C] 2,4‐D more rapidly than did susceptible plants. The half‐life of [ 14 C] 2,4‐D in susceptible plants was 105 h, compared with 22 h in resistant plants. Pretreatment with the cytochrome P450 inhibitor malathion inhibited [ 14 C] 2,4‐D metabolism in resistant plants and reduced the 2,4‐D dose required for 50% growth inhibition (GR 50 ) of resistant plants by 7‐fold to 27 g ha ‐1 , similar to the GR 50 for susceptible plants in the absence of malathion. CONCLUSION Our results demonstrate that rapid 2,4‐D metabolism is a contributing factor to resistance in A. tuberculatus , potentially mediated by cytochrome P450. Metabolism‐based resistance to 2,4‐D could pose a serious challenge for A. tuberculatus control because of the potential for cross‐resistance to other herbicides.