Search IPCM Newsletter Archives

Some of the corn acreage in Missouri has recently been devastated by the heavy rains and flooding that occurred last week, and in some of these fields preemergence corn herbicides had already been applied. This has raised many questions about the replant restrictions of preemergence corn herbicides, as many producers now wish to plant soybeans into fields that had corn and have now been flooded. Table 1 provides a list of some of our most common preemergence corn herbicides and the replant restrictions of these herbicides for corn, grain sorghum, and soybeans. As you can see from this table, soybeans SHOULD NOT be planted into fields where applications of atrazine or an atrazine premix have already been made this season. The label clearly states that soybeans should not be planted until the following year due to the likelihood of soybean injury from residues of atrazine that may still be present in the soil. The average field half-life of atrazine will vary dramatically depending on the soil and environmental conditions experienced, but the Herbicide Handbook published by the Weed Science Society of America lists the average field half-life of atrazine as 60 days. High soil pH's (>7.5) will also slow the degradation of atrazine, along with cool soil conditions. Fortunately, replanting corn or planting grain sorghum into these damaged areas will still be an option where atrazine or most of these atrazine premixes have been applied.

Kevin Bradley
BradleyKe@missouri.edu