Recent shifts in glyphosate-resistant horseweed (Erigeron canadensis L.) emergence patterns and growth types at the field level have generated new management questions. Field experiments investigated the effects of cereal rye and winter wheat, seeded at 67 or 135 kg ha-1, in combination with burndown herbicide strategies or terminated at different times for managing horseweed in no-tillage soybean. In absence of effective herbicides, fall-planted cereal cover crops reduced horseweed biomass up to 70 and 33% at cover termination and five weeks after soybean planting, respectively. Integrating effective herbicide strategies improved horseweed suppression and soybean yield. Delaying termination by Planting Green improved horseweed suppression through the time of postemergence application. Additional field experiments evaluated the effects of termination timing and herbicide combinations for cereal rye termination. Glyphosate applied at 1,267 g ae ha-1 to cereal rye at early (Feekes 6) or late (Feekes 10.5) growth stages effectively terminated cereal rye. The addition of dicamba to glyphosate applied late, or clethodim alone provided less control. All herbicide combinations tested, with the exception of those which included metribuzin, provided similar control to glyphosate alone. In controlled environment experiments, a vernalization period following imbibition of water, but prior to germination, induced horseweed bolting at emergence. Additionally, bolted type horseweed in glyphosate-resistant populations was less sensitive to glyphosate than rosette type. This research provides growers strategies for managing horseweed and insight into the recent glyphosate-resistant horseweed emergence and growth type phenomena observed in the field.
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