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Title: WEED POPULATION RESPONSES TO WEED CONTROL PRACTICES. II. RESIDUAL EFFECTS ON WEED POPULATIONS, CONTROL, AND SOYBEAN YIELD

Author
item Buhler, Douglas - Doug

Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/10/1999
Publication Date: N/A
Citation: N/A

Interpretive Summary: Despite extensive efforts to eliminate them from agricultural lands, weeds continue to plague farmers. An important characteristic that allows many weeds to survive is their ability to produce large quantities of seed and the ability of those seeds to survive in the soil for long periods of time. Prevention of seed production is viewed as justification for striving for higher levels of weed control than necessary to protect crop yields. The objective of our research was to determine the effect of four years of variable weed control treatments on weed populations, herbicide effectiveness, and soybean yield. Based on the results of this research, it appears that weed control practices that maintain weed populations below yield reducing levels do not impair weed control in future years. Keeping plots free of weeds for four years greatly reduced weed densities, but enough weeds remained to reduce soybean yields by 22% or more. Therefore, it seems questionable to attempt to maintain weed-free conditions in a soybean/corn system. Appropriate weed control practices appear to be those that maintain crop yield and quality while keeping weed densities at manageable levels. This research is valuable in the development of integrated weed control practices that minimize herbicide use while maintaining weed populations at manageable levels. It will help farmers make better decisions about controlling weed populations that are below levels that reduce crop yield but raise concerns over seed production.

Technical Abstract: Weed populations, weed control, and soybean yields were affected by a 4-year-history of differential weed control tactics in field experiments initiated under high and low weed densities. In the high density experiment where giant foxtail dominated the initial weed population, plots that were kept weed free for 4 years averaged 24 giant foxtail plants m-2 compared with 200 to 600 plants m-2 following various herbicide and mechanical weed control treatments. The effectiveness of imazethapyr differed among the treatment histories. In the case of plots with a history of mechanical control, the density of weeds escaping control were sufficient to reduce soybean yield compared with plots that were kept weed-free during the same period. In the experiment with low initial weed densities, weed control history had less of an effect on weed densities and there was no effect on weed control history on the effectiveness of imazethapyr or soybean yields in imazethapyr-treated plots. Differences in densities of individual annual broadleaf species also developed in response to treatment history in both experiments. Weed-free conditions for 4 years greatly reduced weed densities. However, when left uncontrolled weeds still reduced soybean yield by 22% in the low density experiment and 51% in the high density experiment.