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Herbicide Carryover This Spring

By Micheal D.K. Owen

Given the dry conditions that prevailed Iowa in 2023, there will be herbicide carryover in 2024. Suspect herbicides that are likely to carryover include, but are not limited to atrazine, clopyralid, and the various HPPD inhibitor herbicides. Specifically, the accuracy, evenness, and timing (i.e., later in the season) of the 2023 herbicide applications can increase the potential for interactions with the 2024 herbicides.

However, just because herbicide residues exist in the soil does not mean that crops will succumb to herbicide injury. If spring conditions are not stressful to the 2024 crop, herbicide injury may not occur. Extremes in soil moisture and/or temperatures that cause crop stress increase the chances that herbicide carryover injury will occur. Stress from herbicides applied in 2024 can also additively cause additional stress resulting in herbicide carryover symptoms.

Generally, herbicide carryover is seen in isolated areas in fields. Areas with low organic matter, high pH, or where herbicide rates are high due to application issues are likely to have herbicide carryover injury. While bioassay testing is mentioned as a tactic to better understand the potential for herbicide carryover, given the spotty nature of areas where herbicide carryover is found, the accuracy of the bioassays is questionable. Often the results are false positives or negatives.

Several strategies will help reduce the risk of herbicide carryover issues in 2024.

  • Planting into favorable conditions, specifically soil temperatures and moisture, will lessen stress on the developing crop and will help keep herbicide carryover symptoms from developing.
  • Timely herbicide applications this spring with correct application rates and evenness, avoiding overlaps, will be extremely helpful in avoiding problems from herbicides applied in 2023.
  • Recognize historical information about fields and plant those fields with past herbicide carryover issues later when conditions improve resulting in less stress on the crop seedlings.
  • Fields that had later herbicide applications in 2023 and were treated with the aforementioned “suspect” herbicides should be planted later.
  • Avoid treating fields in 2024 with herbicides of similar types of those in 2023 (i.e., atrazine in 2023 and metribuzin in 2024).

While the risk of herbicide carryover is quite widespread across Iowa due to the persistent dry conditions, the concern in any individual field depends on the local environment, as well as past and current management practices. Use the above strategies to reduce the risk of an undesirable crop response this spring.

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