EFFECTIVENESS AND ECONOMICS OF REPEATED SEQUENCES OF HERBICIDES FOR CANADA THISTLE (CIRSIUM-ARVENSE) CONTROL IN REDUCED-TILL SPRING WHEAT (TRITICUM-AESTIVUM)

Several postemergence herbicides applied in fall for 2 yr either alone or followed by other spring-applied herbicides for 4 yr reduced densities of Canada thistle (Cirsium arvense (L.) Scop.) infesting reduced-till spring wheat (Triticum aestivum L.). However, fall-applied herbicides for 2 yr had little value for maintaining long-term Canada thistle control, unless supplemented by in-crop herbicide treatment. Neither fall-applied dicamba at 1.7 or 2.2 kg ha-1, glyphosate at 1.7 kg ha-1 plus nonionic surfactant, nor 2,4-D at 1.7 kg ha-1 applied each of 2 yr kept Canada thistle densities below that of the untreated check through year five. Spring-applied chlorsulfuron at 30 g ha-1 plus nonionic surfactant for each of four consecutive years reduced Canada thistle density in wheat to the same extent as fall-applied dicamba followed by chlorsulfuron applied in wheat. Fall herbicide treatments, with or without in-crop herbicide treatments, were economically risky and were seldom profitable. The relative ranking of farmer preference for five treatments common to two trials was similar: untreated check greater-than-or-equal-to spring-applied chlorsulfuron at 30 g ha-1 greater-than-or-equal-to fall-applied dicamba at either 1.7 or 2.2 kg ha-1 (rank reverses between trials 1 and 2) greater-than-or-equal-to fall-applied dicamba at 2.2 kg ha-1 followed by spring-applied chlorsulfuron at 30 g ha-1. The only treatments that were preferred to the untreated check were both spring-applied 2,4-D at 560 g ha-1 and fall-applied 2,4-D at 1.7 kg ha-1 in trial 1 and both spring-applied (4-chloro-2-methylphenoxy) acetic acid (MCPA) plus bromoxynil 280 plus 280 g ha-1, respectively, and spring-applied 2,4-D plus clopyralid at 280 plus 70 g ha-1, respectively, in trial 2.