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 (GR50 ) of resistant plants by 7-fold to 27 g ha-1 , similar to the GR50 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. © 2017 Society of Chemical Industry.

Enhanced atrazine degradation is widespread across the United States.

BACKGROUND: Atrazine (ATZ) has been a key herbicide for annual weed control in corn, with both a soil and post-emergence vegetation application period. Although enhanced ATZ degradation in soil with a history of ATZ use has been reported, the extent and rate of degradation in the US Corn Belt is uncertain. We show that enhanced ATZ degradation exists across much of the country. RESULTS: Soils from 15 of 16 surveyed states had enhanced ATZ degradation. The average ATZ half-life was only 2.3 days in ATZ history soils, compared with an average 14.5 days in soils with no previous ATZ use, meaning that ATZ degrades an average 6 times faster in soils with previous ATZ use. CONCLUSION: When ATZ is used for several years, enhanced degradation will undoubtedly change the way ATZ is used in agronomic crops and also its ultimate environmental fate. © 2017 Society of Chemical Industry.