Volunteer crops resistant to glyphosate and other herbicides pose a potential problem for Pacific Northwest (PNW) growers that rely on glyphosate for control of volunteer crops and weeds during fallow and before planting. Herbicides for control of volunteer herbicide-resistant wheat and canola in PNW conservation tillage systems were evaluated during 2000 and 2001 near Ralston, WA, and Moscow, ID. Paraquat diuron controlled glyphosate- and imidazolinone-resistant wheat ≥90%, and glyphosate controlled imidazolinone-resistant wheat 88 to 96% 14 d after treatment (DAT). Glyphosate- and imidazolinone-resistant wheat were controlled only 58 to 85% with quizalofop-P and clethodim 14 DAT. By 21 DAT, imidazolinone-resistant wheat control with clethodim and quizalofop-P was ≥93%, but the longer time period required for control to reach an acceptable level could increase disease and insect problems associated with volunteer wheat. Volunteer glyphosate-resistant canola was controlled 92 and 97% 14 DAT and 76 and 98% 21 DAT with paraquat and paraquat diuron, respectively. Treatments that contained glyphosate controlled imidazolinone- and glufosinate-resistant canola >84% 14 DAT. By 21 DAT, control of imidazolinone- and glufosinate-resistant canola was 94 to 98% with paraquat diuron and all glyphosate treatments, except glyphosate–isopropylamine salt (IPA) glufosinate (88 to 93%) and glyphosate-IPA paraquat (67 to 85%). In these studies, paraquat diuron was the best alternative to glyphosate for controlling volunteer herbicide-resistant wheat and canola.

Nomenclature: Clethodim; diuron; glufosinate; glyphosate; imidazolinone; paraquat; quizalofop-P; canola, Brassica napus L. ‘DKL 27-20’, ‘Phoenix’, ‘Pioneer 45A71’; wheat, Triticum aestivum L. ‘Bobwhite’, ‘FS-4 IR’.

Additional index words: Direct seeding, glufosinate resistant, glyphosate resistant, green-bridge, imidazolinone resistant.

Abbreviations: ACCase, acetyl coenzyme A carboxylase; DA, diammonium salt; DAT, days after treatment; HRC, herbicide-resistant crops; IPA, isopropylamine salt; PNW, Pacific Northwest.