LAUSR

3265 AbstrAct Palmer amaranth (Amaranthus palmeri S. Watson) resistant to atrazine [6-chloro-N-ethyl-N’-(1-methylethyl)1,3,5-triazine2,4-diamine] and 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicides was confirmed in a seed corn (Zea mays L.) production field in Nebraska, in 2014. Neither atrazine nor HPPD inhibitors (mesotrione [2-(4-mesyl2nitrobenzoyl)-3-hydroxycylohex-2-enone], tembotrione {2-[2-chloro-4-(methylsulfonyl)-3-[(2,2,2-trif luoroethoxy) methyl]benzoyl]-1,3-cyclohexanedione}, or topramezone {[3-(4,5-dihydro-3-isoxazolyl)-2-methyl-4-(methylsulfonyl)phenyl](5-hydroxy-1-methyl-1H-pyrazol-4-yl)methanone}) applied post-emergence were able to control resistant Palmer amaranth even at greater than label rates. However, their tank mixtures even at lower than the label rate provided more than 90% control under greenhouse and field conditions. The objectives of this study were to investigate the effect of atrazine on mesotrione absorption and translocation when tank mixed or vice versa in atrazineand HPPD inhibitor-resistant Palmer amaranth from Nebraska. Tank mixing commercial formulation of atrazine at 560 g ha–1 increased 14C-mesotrione absorption to 51% compared to 39% with 14C-mesotrione alone. However, 14C-atrazine absorption or translocation was not affected by mesotrione at 26 g ha–1 in the tank mixture. Similarly, mesotrione did not affect the metabolism of 14C-atrazine in resistant or susceptible plants when tank mixed compared to 14C-atrazine applied alone. Increased absorption of mesotrione when tank mixed with atrazine could be one of the reasons of atrazine and mesotrione synergism besides their biochemical interaction in the atrazineand HPPD inhibitorresistant Palmer amaranth biotype from Nebraska.