Unintentional herbicide resistance gene stacking in canola may alter the sensitivity of volunteers to herbicides of alternative modes of action commonly used for their control. Greenhouse experiments were conducted to investigate the response of three single-herbicide–resistant (HR) cultivars (glyphosate, glufosinate, imidazolinone), one non-HR cultivar, and seven multiple (double or triple)–HR experimental lines to 2,4-D (amine and ester), MCPA ester, and metribuzin applied at the two- to three-leaf stage and of one non-HR and four HR cultivars (glyphosate, glufosinate, imidazolinone, bromoxynil) to 2,4-D amine applied at two growth stages (two- to three-leaf stage and five- to six-leaf stage). All canola cultivars or lines treated at the two- to three-leaf stage responded similarly to increasing doses of each of the three herbicides. At the five- to six-leaf stage, however, the bromoxynil HR cultivar was less sensitive to 2,4-D than the other cultivars. The results of this study suggest that canola with multiple-herbicide–resistance traits does not differ from cultivars that are non-HR or single HR in its sensitivity to herbicides commonly used to control volunteers. All volunteers, whether non-HR, single HR, or multiple HR, should be treated when plants are most sensitive to herbicides (two- to four-leaf stage) to reduce their interference against crops and their perpetuation of gene flow.

Nomenclature: bromoxynil; 2,4-D; glufosinate; glyphosate; MCPA; metribuzin; canola, Brassica napus L.