GIANT FOXTAIL (SETARIA-FABERI) RESISTANCE TO ARYLOXYPHENOXYPROPIONATEAND CYCLOHEXANEDIONE HERBICIDES

A giant foxtail (PCW1) population putatively resistant to fluazifop-P and sethoxydim was identified in a carrot, onion, and corn cropping sy stem in Wisconsin during 1991. Previous, extensive use of fluazifop an d fluazifop-P over several years imposed a high level of selection int ensity on grass weeds. In a field experiment, fluazifop-P, sethoxydin, and quizalofop at recommended dosages resulted in 58, 53, and 45% pla nt survival, respectively, within the PCW1 giant foxtail population; n o plants survived treatment with clethodim or nicosulfuron, and few pl ants survived treatment with imazethapyr at recommended dosages. Based on shoot dry biomass reduction in greenhouse experiments, a PCW1 gian t foxtail biotype had 16-, > 9-, 4.9-, and 4.2-fold resistance to flua zifop-P, diclofop, quizalofop, and fenoxaprop, respectively, relative to a giant foxtail (AC1) accession that was susceptible to aryloxyphen oxypropionate (APP) and cyclohexanedione (CHD) herbicides. The PCW1 bi otype had 134-fold resistance to sethoxydim and slight and inconsisten t resistance to clethodim. The PCW1 biotype and AC1 accession were equ ally susceptible to imazethapyr, linuron, and oxyfluorfen. Based on pl ant survival, a PCW1 giant foxtail accession had 25- acid > 143-fold r esistance to fluazifop-P and sethoxydim, respectively, relative to the AC1 accession. The selection intensity associated with repeated use o f fluazifop and fluazifop-P over 5 yr contributed to the cross-resista nce of PCW1 giant foxtail to APP and CHD herbicides.