Effectiveness and economics of repeated sequences of herbicides for Canada thistle (<i>Cirsium arvense</i>) control in reduced-till spring wheat (<i>Triticum aestivum</i>)

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 infesting reduced-till spring wheat, unless supplemented by in-crop herbicide treatment.

Abstract

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 ≥ spring-applied chlorsulfuron at 30 g ha −1 ≥ fall-applied dicamba at either 1.7 or 2.2 kg ha −1 (rank reverses between trials 1 and 2) ≥ 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.Key words: Bromoxynil + MCPA, chlorsulfuron, dicamba, glyphosate, 2,4-D