No. 9 Article 4/May 23, 2008

Weed Control in Corn

The current Illinois corn crop exhibits a broad array of sizes and growth stages, ranging from areas of central and west-central Illinois with plants large enough to be treated with postemergence herbicides to southern Illinois stands requiring replanting due to overabundant precipitation. The following comments focus mainly on replanting corn where precipitation has been excessive.

Questions have been asked about what options are available to control emerged corn from the first planting before the field is replanted to corn. Tillage is effective for control of the first planting and introduces very little risk of injury to the replanted corn. However, some farmers may not want to disturb the soil prior to replanting and have asked if there are herbicides that can effectively control the first planting and allow replanting to proceed soon after application. If herbicides will be used, they should be applied before the fields are replanted.

In 2006 and 2007 field research, we evaluated several herbicides and herbicide combinations to control a first-planting corn stand prior to replanting. Many treatments looked promising when evaluated 7 days after application, but by 14 days after application levels of control had decreased dramatically as the treated corn began to recover. A tank-mix of Gramoxone Inteon plus Sencor was one of the most effective "labeled" treatments across both years, but corn control 14 days after application did not exceed 85% either year. Treatments containing glufosinate or isoxaflutole were not effective at controlling corn from the first planting.

Glyphosate is very effective for controlling emerged corn, and there is no waiting interval between application and replanting specified on the label. Obviously, however, glyphosate will not control emerged glyphosate-resistant corn, so alternative herbicides or tillage will be needed. The postemergence grass control herbicides used in soybean can effectively control emerged corn, but the product labels specify rotational intervals between application and planting corn. The labels of Poast Plus, Select, Fusion, Fusilade DX, and Assure II indicate rotational intervals of 30, 30, 60, 60, and 120 days, respectively, between application and corn planting.

Recently, SelectMax received 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. Applications should include a nonionic surfactant and ammonium sulfate, 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 what is labeled for this type of application, that in-field overlaps be avoided, and that the 6-day waiting interval be fully expired before treated fields are replanted.

Many fields that will be replanted were initially treated with one or more soil-residual herbicides. Is there an interval between when a herbicide was applied and when corn replanting can occur? For soil-applied and most postemergence corn herbicides, replanting can proceed whenever field conditions are feasible. However, a small number of postemergence corn herbicides require intervals between application and replanting. For example, if a cornfield previously treated with Spirit, Status, NorthStar, Permit, or Yukon is lost and must be replanted, an interval of 4 weeks, 7 days, 14 days, 1 month, or 1 month, respectively, must elapse between herbicide application and corn replanting.

Can fields previously treated with a soil-applied herbicide be retreated with the same product before the corn is replanted? The answer varies by product and in some instances with whether the initial soil application was broadcast or banded. For example, the Radius label allows for only one application per season, so reapplying Radius before replanting fields previously treated with it is not allowed. Other product labels actually prohibit reapplying the product before replanting. For example, even though Dual II Magnum can be applied preplant, preemergence, or postemergence, the label specifies that "if a crop treated with Dual II Magnum alone is lost any crop on the label may be replanted immediately. Do not make a second broadcast application of Dual II Magnum." Other product labels may restrict the number of times a product may be applied per season but do not preclude retreating a field prior to replanting corn. For example, the Micro-Tech label allows for only two applications per year but does not prohibit retreating a field prior to replanting. Table 1 provides information from corn herbicide labels regarding reapplication, maximum seasonal use rates, and crop rotational intervals. If you elect to make a second application of a particular corn herbicide, keep in mind that many product labels indicate a maximum per-acre rate that can be applied during one growing season. If a particular product can be applied after corn emergence, the maximum rate per acre and per season would account for this type of application as well. Refer to "Weed Control in Corn and Wet Field Conditions" in issue 6 of the Bulletin (May 2, 2008) for a list of soil-residual corn herbicides that can be applied after corn emergence.--Aaron Hager

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