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Issue No. 6, Article 4/May 14, 2010

Replanting Corn: How to Control Corn Plants from the Initial Planting

The following information first appeared in a 2009 edition of the Bulletin. The recent heavy rains in some areas of Illinois may lead to corn replanting, so we decided to publish related recommendations again this year.

Certain areas of Illinois recently received excessive amounts of precipitation, and it appears likely that some corn replanting will occur when field conditions improve. Losses of the initial corn stand will likely range from complete to partial, so farmers may need to implement practices to control any remaining corn plants before replanting. What options are available to control emerged corn from the first planting?

Tillage can effectively remove corn plants from the first planting regardless of their herbicide sensitivity or resistance characteristics remaining, although more than one pass may sometimes be needed. This option introduces very little risk of injury to the replanted corn, unlike some herbicide alternatives described in the following paragraphs. Tillage also can effectively control any weeds that might have emerged in the first planting, providing weed-free conditions for planting. However, for myriad reasons farmers may not want to disturb the soil prior to replanting and thus may look to herbicide alternatives. If herbicides will be used, they should be applied before fields are replanted.

Glyphosate is very effective at controlling existing stands of (sensitive) corn. No waiting interval between application and replanting is specified on the label, but overall control may be improved if at least 24 hours elapse. Glyphosate would also provide control of most emerged weeds, allowing replanting into weed-free conditions. However, glyphosate obviously would not control existing stands of glyphosate-resistant corn (or any glyphosate-resistant weeds that might already have emerged), so alternative herbicides would be required.

Poast, Poast Plus, Fusion, Fusilade, Select, and Assure II are effective for controlling volunteer corn in soybean (including volunteer glyphosate-resistant corn), but each product label includes an interval that must elapse between application and rotation to or replanting with grass crops such as corn. These intervals range from 30 days (Poast, Poast Plus, Select) to 60 (Fusion, Fusilade) to as many as 120 (Assure II), making these products unlikely choices for this particular use.

We have evaluated several different herbicides or herbicide combinations for control of an existing stand of corn. Some results from this research, conducted in 2006 and 2007, are summarized in Table 1. Treatments that included glufosinate, paraquat, or isoxaflutole often appeared to be providing good control 7 days after application, but corn frequently recovered by the 14th day after application.

SelectMax has a supplemental label for control of an existing stand of glyphosate-resistant corn prior to replanting field corn. The label allows applications of 6 fluid ounces per acre for control of glyphosate-resistant field corn up to 12 inches tall. We have limited research experience with this particular rate of SelectMax, but we have observed good to excellent control of corn with a 4-ounce rate of SelectMax. Applications should include NIS and AMS (do not use a COC or MSO in this particular use), and care must be taken to avoid in-field overlaps, or excessive injury to the replanted corn may occur. Do not replant fields treated in this way sooner than 6 days after application. Previous research has shown that clethodim, the active ingredient in SelectMax, has some soil persistence and can cause significant damage to corn when applied at higher rates prior to planting. It is strongly advised that rates not exceed the rate labeled for this type of application, that in-field overlaps be avoided, and that the six-day waiting interval be fully expired before treated fields are replanted.--Aaron Hager

Aaron Hager

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