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Issue No. 8, Article 1/May 15, 2009

Applying Soil-Residual Herbicides After Corn Planting and Emergence

Soil-residual herbicides, an important component of an integrated weed management program, reduce the potential for corn yield loss caused by early-season weed interference and help reduce the selection intensity for herbicide-resistant weed biotypes. Most often these products are applied shortly before or after planting but before corn emerges. Given the planting challenges imposed on Illinois corn farmers by this season's unfavorable weather, it is altogether likely that some, even many, fields were planted before a soil-residual herbicide was applied. In some instances the corn has not yet emerged, whereas in others it has. If you still intend to apply a soil-residual herbicide, take note of these general considerations:

Corn is planted but not yet emerged:

  • Closing the seed furrow can be difficult if planting occurs under wet soil conditions. This by itself can lead to establishment problems, but if a soil-residual herbicide will be applied soon after planting, an open seed furrow provides an avenue for direct contact of the herbicide with the seed. Labels of many soil-residual corn herbicides warn that severe corn injury can result if the herbicide comes in direct contact with the seed.
  • Be especially cautious about making preemergence applications to fields where the corn is within a day or two of emerging--especially if nonselective herbicides are added to control existing vegetation--or with soil-residual herbicides that should not be applied after crop emergence. Even if the crop hasn't fully emerged or isn't yet visible from the road, small cracks or other openings in the soil surface may allow the herbicide to come into direct contact with the emerging coleoptile. Labels of most soil-residual herbicides prohibit nitrogen fertilizer as the herbicide carrier if corn has begun to emerge.
  • If several days have elapsed between planting and the application of the soil-residual herbicide, weed seedlings could be close to emerging from the soil and, depending on the herbicide, may or may not be adequately controlled by the soil-residual herbicide.

Corn is planted and has emerged:

  • Be aware that many, but not all, soil-residual corn herbicides can be applied after corn emergence. Not all of them will control emerged weeds, so additional management procedures (such as the use of a rotary hoe or the addition of a herbicide that has postemergence activity) may be needed in situations where weeds also have emerged. Table 1 summarizes information about postemergence applications of the more traditional soil-applied corn herbicides. Consult the respective product label for additional information, such as the need for a tank-mix partner or spray additive to improve control of existing weeds.

  • Also be aware of the potential for enhanced corn injury if a soil-residual herbicide is applied during periods of crop stress, such as stress caused by excessive soil moisture or by cool air or soil temperatures. Depending on the herbicide, tank-mixing other products or including various types of spray additives may be necessary to control existing weeds, but this may also increase the potential for corn injury.

Whatever the scenario, always remember that soil-residual herbicides need to be moved into the soil solution to be available for uptake by weed seedlings. A herbicide that remains on the soil surface after application and is not moved into the soil profile by precipitation or mechanical incorporation may not provide adequate weed control.--Aaron Hager

Aaron Hager

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