Field experiments were conducted at three locations in 2003 and 2004 to examine clethodim and quizalofop-P efficacy on spring wheat seedlings when applied alone or in tank mixtures with herbicides used to control broadleaf weeds. Clethodim at the recommended rate of 30 g/ha reduced spring wheat biomass by 63 to 98% and was only >90% in three of six site years. In contrast, quizalofop-P at the recommended rate of 36 g/ha reduced wheat biomass >90% in all cases. Clethodim or quizalofop-P could be tank mixed with 2,4-D ester, bromoxynil, or bromoxynil plus MCPA ester with little risk of reduced efficacy on wheat. However, 2,4-D amine was highly antagonistic to both herbicides. The commercial mixture of thifensulfuron plus tribenuron reduced clethodim, but not quizalofop-P, efficacy on wheat. Herbicide options exist for simultaneous control of volunteer glyphosate-resistant canola and glyphosate-resistant wheat if the latter technology were to be commercialized in the future.

Nomenclature: Bromoxynil; clethodim; 2,4-D; MCPA; quizalofop-P; thifensulfuron; tribenuron; canola, Brassica napus L.; wheat, Triticum aestivum L.

Additional index words: Antagonism, glyphosate-resistant crops, herbicide interaction, herbicide mixture, volunteer management.

Abbreviations: ACCase, acetyl-CoA carboxylase (EC 6.4.1.2).