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Issue No. 4, Article 4/April 29, 2010

Applying Soil-Residual Herbicides After Corn Emergence

The rapid progress in planting corn in Illinois is quite different from what many farmers experienced in 2008 and 2009. A look back at the Bulletin articles about this time last year indicates we were by and large still waiting for field conditions to improve so planting could begin, and weed vegetation was increasingly robust. In stark contrast, this season we've already discussed how to improve the performance of soil-residual herbicides through mechanical incorporation or timely precipitation following application.

It's quite possible that the rapid planting progress has resulted in some fields' being planted before a planned soil-residual herbicide could be applied. If the corn has yet to emerge, the application could proceed as originally planned. But what if the corn has begun to emerge and the soil-residual herbicide has not yet been applied? Can the application go on as planned, or will a different product be needed?

The answer depends on the herbicide in question. Many but not all herbicides that are most often applied before corn planting or emergence can be applied after the corn has emerged.

Even if a soil-residual herbicide can be applied after crop and weed emergence, not all soil-residual herbicides will control emerged weeds, so additional management procedures (such as adding a herbicide with postemergence activity) may be needed in situations where weeds also have emerged. Table 1 summarizes information about post-emergence applications of the more traditional soil-applied corn herbicides. Consult the respective product label for additional information, such as the need for tank-mix partners or spray additives to improve control of existing weeds.

Farmers are also cautioned about the potential for enhanced corn injury if these products are applied during periods of crop stress, such as stress caused by excessive soil moisture and 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.

Regardless of 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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