Field experiments were conducted to evaluate if fall-planted cereal rye in combination with narrow row soybean improved glyphosate-resistant (GR) horseweed management. At the time of a postemergence herbicide application (POST), horseweed biomass was 71 to 90% lower when soybean was planted into cereal rye, regardless of termination time, compared with no cover across all row widths. Planting green or narrow row soybean suppressed horseweed through soybean harvest and integrating an effective POST herbicide improved control. Additional field experiments found that in the absence of an effective POST herbicide, horseweed biomass was 42 and 81% lower by planting green or applying a residual herbicide compared with no cover, respectively, at soybean harvest. Similarly, planting soybean in 19 cm rows reduced horseweed biomass compared with 38 and 76 cm rows. In the greenhouse, shade levels from 35 to 92% reduced rosette and upright horseweed biomass 31 to 99% compared with the upright-type grown under 0% shade. Greater reductions occurred under 69 and 92% shade. Differences in glyphosate sensitivity between the rosette and upright horseweed growth types were not due to absorption, translocation, or total glyphosate retention; however, glyphosate retention was 21 and 18% lower on a per weight and area basis for the upright growth type. This diluted concentration may contribute to increased glyphosate tolerance found in the upright growth type. However, other factors such as differences in EPSPS gene expression may also help explain differential sensitivity if a target-mutation is discovered. This research provides growers strategies for managing horseweed and insight into potential growth type differences.
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