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Herbicide Considerations When Replanting

June 15, 2001
Excessive precipitation over the past two weeks has resulted in many new "ponds" appearing in fields where corn or soybean once grew. Replanting these areas will likely be done as soon as soil conditions allow. Previous herbicide applications should also be considered as replanting decisions are made. Nearly all corn and soybean herbicide labels list rotational intervals that should be followed to reduce the likelihood of rotational crop injury. The vast majority of corn herbicides (soil-applied or postemergence) allow corn to be replanted immediately following application. However, the NorthStar (14 days), Beacon (14 days), Spirit (4 weeks), Permit (1 month), and Celebrity Plus (1 week) labels list corn replanting intervals. Table 1 is reproduced from the 2001 Illinois Agricultural Pest Management Handbook and contains label information with respect to corn herbicide recropping restrictions.

Being the middle of June, it appears likely that some producers who plan to replant drowned areas of cornfields might be considering replanting these areas with soybean. Planting soybean in cornfields previously treated (this season) with atrazine is not a recommended procedure, and most labels of herbicides containing atrazine actually restrict planting soybean in fields where atrazine has been applied. From a practical standpoint, it's highly likely that some planting of soybean into atrazine-treated fields will occur. If this happens, it's a gamble! Like most activities of chance, there are odds of success and odds of failure. What can you do to increase the odds of success?

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 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 suggested 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 to capture as much sunlight as possible.

There are many factors to consider when making replanting decisions. Planting soybeans in fields previously treated with atrazine is very risky, as soybeans are sensitive to atrazine. 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 favor enhanced atrazine availability for plant uptake. At this point in 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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