Herbicide Carryover Concerns

Due to the drought conditions in parts of the Midwest in 2023, there is some concern for potential soil-active herbicides to carryover in 2024. 

📸: @Elmar Gubisch via Canva/Featuring drought damage in corn.

How Herbicides Work in the Soil Under Normal Conditions

Under normal conditions following an application, an equilibrium is created where a majority of a herbicide is adsorbed to soil organic matter and clay while the remainder is free in the soil water solution. The herbicide in the soil water solution is available for absorption by roots, resulting in weed control. This balance of free vs. bound herbicide is necessary to keep a dose available for weed control in the soil solution, while the portion bound to the soil organic matter and clay is not available for rapid herbicide breakdown by soil microorganisms or leaching beyond the weed root zone. Eventually, as more herbicide is released into the soil solution, weed control is maintained, and the bound herbicide is depleted, leaving soil relatively free of any herbicide residue.

What Changes Under Drought Conditions?

We have all seen situations where the soil is very dry and weed control suffers, as there is little herbicide in the soil water solution for weeds to absorb. When this happens, almost all the herbicide is tightly bound and not available for weed control or breakdown. The main breakdown mechanism for most herbicides is by microbes, which are soil fungi and bacteria. Microbes require soil moisture to be active and can only access herbicides that are available in the soil solution. If dry conditions persist, a relatively high amount may be bound in the soil going into the next season.   

📸: @Drbouz via Canva

What To Look for Following a Drought

Herbicide carryover is generally noticed within several weeks after crop emergence when roots can absorb the herbicide residue. Patterns will appear that may be soil type-related and in any compacted areas, including end rows and spray overlaps. Under very dry conditions, there are herbicides with certain characteristics that are of more concern than others. Herbicide characteristics that can enhance carryover potential following drought conditions are:

• The rotational crop is naturally very sensitive to the herbicide.

• The herbicide has a relatively long herbicide half-life or breakdown period under normal conditions.

• The herbicide half-life is extended by extreme soil pH and soil type.

Corn herbicides used in 2023 fields and rotated to soybeans in 2024:

• HPPD Herbicides: Labels of group 27 (HPPD) herbicides or premixes containing HPPD herbicides range from an 8 to 10-month half-life rotational restriction to soybean, indicating a relatively long half-life. In addition, the half-life of HPPDs is greatly increased in soils with a pH greater than 7.5. HPPD carryover symptoms in soybeans are characterized by stunted soybeans with white leaves in the new growth.

• Atrazine: Labels of group 5 (triazine) herbicide or premixes containing atrazine call for soybeans to be planted the next year. Watching use rates in corn is very important when following with soybeans. Atrazine breakdown is also slowed in high-pH soils. One look-alike symptom of atrazine carryover to soybean is iron deficiency chlorosis (IDC), but they can be distinguished from each other as atrazine carryover will affect older leaves, while IDC affects new growth.

• Clopyralid: Labels of the group 4 clopyralid herbicide or premixes containing clopyralid require 10.5 to 18 months plantback to soybeans, depending on organic matter and rainfall. Soybeans are very sensitive to small amounts left in the soil. Clopyralid carryover to soybean has a typical growth regulator strapping or puckering of soybean leaves. It should be noted that 2,4-D resistant soybeans are not tolerant to clorpyralid carryover.

📸: BASF Contributing Writer/Featuring Clopyralid carryover in soybean.

Soybean herbicides used in 2023 fields rotated to corn in 2024:

• Fomesafen: Unlike other group 14 herbicides that have a relatively short half-life, the label for fomesafen or fomesafen containing premixes requires 10 months plantback to corn. Label rates are regulated by geography. Fomesafen carryover symptoms in corn show a unique veinal chlorosis where the areas between each vein remain green but the veins on the corn leaf turn yellow.

📸: BASF Contributing Writer/Featuring Fomesafen carryover symptoms Veinal Chlorosis of Corn Leaf.

• Imazethapyr:  The label for group 2 imazethapyr or imazethapyr containing premixes requires 9 months of plantback to corn. The soil half-life is greatly increased under low soil pH, especially below 6.0. Imazethapyr carryover injury is visible after V1 corn in the form of stunted plants and yellow or purple corn leaves. One look-alike symptom of imazethapyr is phosphorous deficiency, which can also cause stunting and purple leaves.

Management Practices to Reduce Potential Carryover

You can’t control the weather, but there are things you can do that will reduce the potential for herbicide carryover.

• Do not exceed the maximum labeled rate.

• Do not use multiple applications of the same herbicide or other herbicides with the same mode of action in the same season.

• Obey crop rotational restrictions on the label.

• Avoid late-season herbicide applications.

• Avoid spray pattern overlaps.

• Utilize herbicide-tolerant varieties or traits when appropriate.

• Correct soil pH extremes in your field.

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