Weed population responses to weed control practices. II. Residual effects on weed populations, control, and Glycine max yield

Weed populations, weed control with imazethapyr, and Glycine max yields wer e affected by a 4-yr history of different weed control practices. A range o f chemical and mechanical practices were applied in separate field experime nts initiated under high and low weed densities. In the high-density experi ment, 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 treatm ents. In plots with a history of mechanical control, weeds not controlled b y imazethapyr reduced G. max yield by 340 kg ha(-1) compared with plots tha t 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. f aberi 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 effec t 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-densit y experiment and 51% in the high-density experiment. Differences in densiti es of individual annual broadleaf species also developed in response to wee d control history in both experiments.