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Corn Replanting and Herbicide Considerations

May 24, 2002
With each successive rain, the likelihood of having to replant corn increases. We would like to hope that replanting would need to occur only in small areas of a given field, but with many streams and ditches running over their banks this spring, entire fields may have to be replanted. While there are many agronomic considerations associated with replanting, some weed control/herbicide issues should also be considered.

Hybrid selection for the replanting operation should be taken into account, especially if the field has been previously treated with certain soil-applied or postemergence herbicides that require a particular herbicide-resistant/-tolerant corn hybrid. For example, if a Clearfield corn hybrid was initially planted and received a preemergence application of Pursuit Plus or has already been treated postemergence with Lightning, the replanted corn must also be a Clear-field hybrid. If a nonresistant/tolerant hybrid is replanted instead, the potential exists for the herbicide to cause a great deal of crop injury. If you initially planted a Roundup Ready corn hybrid and have areas that need to be replanted, either replant these areas with a Roundup Ready hybrid or take special precautions during the post-emergence glyphosate application if you replant with a conventional hybrid.

Is there an interval between when a herbicide was applied and corn replanting? For soil-applied corn herbicides, replanting can proceed whenever field conditions are feasible. However, for a small number of post-emergence corn herbicides, there are intervals between application and replanting. For example, if a cornfield previously treated with Spirit, North-Star, Permit, or Yukon is lost due to excessive precipitation and must be replanted, there is a 4-week, 14-day, 1-month, and 1-month, respectively, interval that must elapse between the herbicide application and corn replanting.

While most soil-applied herbicides allow more than one application per season, a few can be applied only once per season. For example, the Epic label indicates that only one application per year can be made; Prowl can be used as a soil-applied or postemergence treatment, but if a previously treated corn crop must be replanted, do not make another application prior to replanting. In instances where small areas of a field will be replanted, some may elect to simply replant without applying any additional herbicide. If, however, 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. The maximum atrazine rate for combined soil-applied and postemergence applications that can be applied per acre per year is 2.5 pounds of active ingredient.

Rotating to Other Crops

For the time being, most producers with areas to replant will opt to replant to corn, but as the season progresses and wet field conditions persist, others may switch to soybeans. If no corn herbicide has been applied, switching to soybeans can be done without having to worry about corn herbicide resi-dues that may cause soybean injury. However, if a corn herbicide has been previously applied, the potential for soybean injury may exist. Table 2 is reproduced from the 2002 Illinois Agricultural Pest Management Handbook and provides label information with respect to corn herbicide recropping restrictions. Several people have already asked about switching to soybeans in fields that were treated previously with atrazine or an atrazine-containing premix. Planting soybean in fields previously treated (this season) with atrazine is not recommended by the University of Illinois, and most labels of herbicides containing atrazine restrict planting soybean in fields where atrazine has been applied.

However, from a practical standpoint, it's highly likely that some planting of soybean into atrazine-treated fields will occur. While (again) this is not a recommended procedure, some of the following practices are offered for consideration.

The persistence of triazine herbicides is strongly influenced by soil pH. As a general rule, as the soil pH increases, so too does the persistence of these herbicides. When soil pH values are 7.0 or greater, the dissipation of triazine herbicides is greatly reduced. So, before you switch to soybeans, it might be worthwhile to check the soil pH.

Anything that can be done to dilute the atrazine in the soil could prove beneficial. The easiest way to achieve this is through tillage. Tillage can help to reduce areas of high atrazine concentration in the soil so that the soybeans may not be exposed to zones of high concentration all at once. Is this strategy practical, given the currently wet conditions? If field conditions are suitable to plant, a tillage pass prior to planting may also be feasible.

Certain soybean varieties are more sensitive than others to the herbicide metribuzin (Sencor). It may also be beneficial to avoid planting metribuzin-sensitive soybean varieties into fields previously treated with atrazine. Contact your soybean seed representative to determine if the soybean variety you intend to plant is overly sensitive to metribuzin.

Along a similar line, producers may want to consider avoiding soybean herbicides containing metribuzin when planting soybeans into fields previously treated with atrazine. Metribuzin belongs to the same chemical family as atrazine, and the added effect of two triazine herbicides may be more than the soybeans can handle. Soybean herbicides containing metribuzin include Sencor, Canopy, Axiom, Boundary, and Domain.

Soybean seed size may also influence tolerance to atrazine. Early research suggests that planting large soybean seeds may be more beneficial than planting smaller seeds in fields treated with atrazine. The larger seed contains more stored food reserves for the seedling to survive on longer before relying on photosynthesis for its food supply.

Finally, producers may want to consider increasing the planting rate slightly to compensate for plants that may be lost due to the atrazine. The later into the growing season the soybean planting occurs, the higher the planting rate adjustment producers may want to consider making in order to capture as much sunlight as possible.

A similar scenario may unfold for cornfields that received a fall application of Princep (simazine). As with atrazine, soybeans can be severely injured by simazine residues, and simazine persistence increases with higher soil pH levels. If fields where a fall Princep application are "greening up" with weed species susceptible to simazine, it may be reasonable that the risk of soybean injury from simazine residues is less than if these species were not present.

There are many factors to consider when making replanting decisions. Planting soybeans in fields previously treated with certain corn herbicides is very risky, as soybeans are very sensitive to some of these herbicides. Many factors contribute to the availability of atrazine in the soil for plant uptake. Those factors that reduce the availability of atrazine can be beneficial for soybean survival. However, other factors (high soil pH) favor enhanced atrazine availability for plant uptake. At this time, it's not possible to predict which factors will predominate.--Aaron Hager and Christy Sprague

Author: Aaron Hager Christy Sprague


The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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