Weed population responses to weed control practices. II. Residual effects on weed populations, control, andGlycine maxyield

Weed populations, weed control with imazethapyr, and Glycine max yields were affected by a 4-yr history of different weed control practices. A range of chemical and mechanical practices were applied in separate field experiments initiated under high and low weed densities. In the high-density experiment, plots kept weed free for 4 yr averaged 24 Setaria faberi plants m −2 compared with 200 to 600 plants m −2 with the various weed control treatments. In plots with a history of mechanical control, weeds not controlled by imazethapyr reduced G. max yield by 340 kg ha −1 compared with plots that were kept weed free during the same period. In the low-density experiment, weed control history had less effect on weed densities. For example, S. faberi densities ranged from 19 plants m −2 for the weed-free plots to 195 plants m −2 with mechanical control. Weed control history had little effect on weed control with imazethapyr or G. max yields in imazethapyr-treated plots. While weed-free conditions for 4 yr greatly reduced weed densities, imazethapyr application still increased G. max yields 22% in the low-density experiment and 51% in the high-density experiment. Differences in densities of individual annual broadleaf species also developed in response to weed control history in both experiments.