Herbicide carryover impact on failed corn

Where corn has failed this year, producers may be thinking about planting wheat this fall. Unless producers had been planning for this rotation all along, they may find that the herbicides they applied to the corn crop will not allow them to plant wheat.

Most corn herbicides, other than atrazine, have a 4.5 month or less rotational restriction when planting back to winter wheat. However, atrazine or any premix herbicide containing atrazine, will be the main herbicides of concern. Words directly from the atrazine label are as follows:

Rotational Crops – All Atrazine uses: (1) Do not rotate back to any crop except corn or sorghum until the following year, or injury may occur (2) If atrazine is applied after June 10, do not rotate with crops other than corn or sorghum the next year, or crop injury may result.

The above statement suggests that if atrazine is used, regardless of rate or whether it is applied preemergence or postemergence, wheat could be injured if planted in the fall following the application.

Atrazine breakdown is clearly affected by soil pH and precipitation. Dry conditions will slow the degradation of atrazine, making it difficult to predict the risk of potential crop injury.

Research under normal conditions suggests that half-life of atrazine in the soil is:

• 39 days at pH 6.8
• 119 days at pH 7.3
• 261 days at pH 8.0

Crop injury to wheat following atrazine application is most likely under high-pH or dry conditions.

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