University of Illinois

No. 10/May 29, 1998

Another Consideration When Replanting

The last edition of this Bulletin contained information about replanting corn fields and the choice of corn hybrids. The heavy rains last week across much of Illinois have again caused many "ponds" to appear in planted corn fields. With June rapidly approaching, producers are beginning to consider replanting entire corn fields, or sometimes only the drowned areas, with soybeans. Previously applied corn herbicides can influence the decision to switch from replanting corn to planting soybeans.

Almost all corn herbicide labels contain information about replanting options should the initial corn crop be lost. Be aware that many corn herbicide labels restrict planting soybeans during the same season the herbicide was applied. Consult herbicide label(s) to determine if soybeans can be planted in fields previously treated with corn herbicides.

We have received a number of questions about planting soybeans in fields previously treated with atrazine. Planting soybeans in corn fields previously treated (this season) with atrazine is not recommended, and most labels of herbicides containing atrazine actually restrict planting soybeans in fields where atrazine has been applied.

From a practical standpoint, it's highly likely that some planting of soybeans into atrazine-treated fields will occur. If this happens, the outcome is a gamble! What can you do to increase your 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 with tillage, which can help 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, Lexone). It may also be beneficial to avoid planting metribuzin-sensitive soybean varieties in 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 the application of soybean herbicides containing metribuzin when planting soybeans in fields previously treated with atrazine. Metribuzin belongs to the same chemical family as atrazine and the combined effect of two triazine herbicides may be more than the soybeans can handle. Products containing metribuzin include Sencor, Lexone, Canopy, Turbo, and Axiom.

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 on which the seedling can survive before it must rely upon 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 that producers may want to consider making.

In short, many factors contribute to the availability of atrazine in the soil for plant uptake. 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, it's not possible to predict which factors will predominate.

Aaron Hager ( and Marshal McGlamery (, Department of Crop Sciences, (217)333-4424