Weed management in continuous corn

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.  

Although many people have rotated corn to soybeans or other crops this year, if you are planting corn following corn there are several weed management issues to consider. Herbicide inputs are a greater concern in a continuous corn system when compared to growing corn in rotation with another crop. This is due primarily to increased crop residue levels as well as weed escapes from the previous year. Increased residue levels can decrease the efficacy of many soil-applied herbicides and provide certain weed species, such as small seeded broadleaf and grass weeds, the opportunity to germinate and compete with corn. In a continuous corn system, it is a good idea to plan to use herbicide programs that account for deficiencies in the previous year’s weed control operations. Fields where weed escapes occurred the prior year can leave hundreds or thousands of weed seeds in the soil seed bank. Using the same herbicide program two years in a row can potentially compound the problem creating greater weed populations and weed control issues in the future. Additionally, many weeds have a relatively long emergence period, which generally makes two-pass weed control programs more successful.

Due to greater residue in a continuous corn system, full rates of soil-applied herbicides are recommended to manage weeds. Total postemergence herbicide applications can be used effectively; however, ensure that weeds are not taller than 4 inches before making applications. Delaying preemergence herbicide applications after corn emergence can extend the residual window of activity and reduce late-season weed emergence. Be sure to follow the precautions given in last week’s
Field CAT Alert when delaying preemergence applications. Several residual preemergence herbicides can also be applied with herbicides that provide foliar activity to control emerged weeds. In general, a planned two-pass herbicide program of preemergence and postemergence weed control strategies will provide the most effective weed control.

Although glyphosate-resistant weeds have not been observed in Michigan row crops, continue to reduce the risk of glyphosate-resistant weeds by varying herbicide modes of action, especially on weeds that can be problematic to control with glyphosate alone. Where glyphosate-resistant corn was grown last year, consider using herbicides with different modes of action on the most problematic weeds to reduce selection pressure for glyphosate-resistant weeds. Herbicide Sites of Action are listed on pages 16-17 in the
2008 Weed Control Guide for Field Crops.

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