The prolonged cool soil temperatures and wet conditions over the last several days slowed the metabolism and emergence of corn plants and allowed more exposure time of corn seed and emerging seedlings to herbicides in soil solution. Prolonged exposure to certain herbicides can lead to injury.

The following is a brief discussion of some of the herbicides and symptoms associated with early-season corn injury during cool wet soil conditions:

Chloroacetamide / Oxyacetamide Herbicides: Chloroacetamide herbicides are widely used for weed management in corn. Examples of herbicides in this class of chemistry are acetochlor (e.g. Degree, Degree Xtra, Harness, TopNotch, Harness Xtra); alachlor (e.g. Micro-Tech or Bullet); dimethenamid-P (e.g. Outlook or Guardsman Max); metolachlor (e.g. Parallel or Stalwart C) and S-metolachlor (e.g. Dual II Magnum, Camix, Charger Max, Cinch, Bicep II Magnum, Lexar, or Lumax). Flufenacet (e.g. Define, Axiom, or Epic) is classified as an oxyacetamide and is similar to chloroacetamide herbicides.

Acetochlor, metolachlor, and S-metolachlor tend to have a greater potential to cause injury compared with similar herbicides, yet they are usually formulated with a safener to limit the risk of corn injury.

Symptoms of corn injury from chloroacetamide and oxyacetamide herbicides include malformed and stunted seedlings with twisted leaves that do not unroll properly. Plants that are severely injured may eventually die because leaves cannot unroll.

Synthetic-Auxin Herbicides. Products containing 2,4-D and dicamba (e.g. Banvel, Clarity, Oracle, or Sterling) are often referred to as synthetic-auxin or growth-regulator herbicides. They can be used as a part of a burndown weed control program in no-tillage corn. Their activity is based primarily on foliar uptake in emerged weeds; however, they have limited soil-residual activity that can sometimes result in corn injury.

Corn injury from synthetic-auxin herbicides occurs during seed germination and resembles symptoms associated with chloroacetamide and oxyacetamide herbicides. Synthetic- auxin herbicides often cause abnormalities of both shoots and roots of corn plants; whereas, chloroacetamide and oxyacetamide herbicides tend to affect only the shoot growth.

Products containing 2,4-D generally should not be used during the period from 7 to 14 days prior to planting until 3 to 5 days after planting and before corn emerges. Products containing dicamba may injure corn if seed is planted less than 1.5 inches below the surface.

Isoxaflutole: Injury from products containing isoxaflutole (e.g. Balance Pro, Epic, or Radius) is seldom an issue in Kentucky but has occurred when soil conditions were cold and wet and weather was cloudy for several days following planting. Corn injury symptoms associated with isoxaflutole range from minor yellowing or complete bleaching of plants to significant stand loss. Isoxaflutole does not directly affect roots but may indirectly limit root growth by inhibiting photosynthetic activity of plants. Injury from isoxaflutole is usually short lived but can cause yield loss if corn stands are reduced.

Pendimethalin: Preemergence applications of pendimethalin (e.g. Prowl H2O or Pendimax) can result in significant injury where corn seed are exposed to the herbicide. This typically occurs when corn is planted shallow (less than 1.5 inches deep) or when the seed furrow is not completely closed or sealed. Injured corn plants have stunted roots with swollen tips. Corn stands can be reduced in severe cases. Surviving plants are usually stunted and may turn purple, particularly during prolonged cool soil conditions after plant emergence.

Flumetsulam: Extended cold wet conditions (soil temperature below 500 F and excessive rainfall with wet soil conditions) increase the chance of injury from flumetsulam (Python, Hornet) during germination and early development of corn. Injury symptoms caused by flumetsulam are similar to those caused by other ALS-inhibiting herbicides and include yellowing of leaves and /or stunted shoots and a reduced root system.

While corn injury is a risk under cool and wet environmental conditions, there is no assurance it will occur. Fields need to be monitored to assess the corn stands to determine if symptoms are present. Corn may outgrow injury, however it will likely require several days of warm temperatures for plants to recover. If it is decided to replant, the surviving corn plants will need to be controlled. See information in Corn and Soybean Newsletter (Vol. 7 Issue 4) for controlling corn as a weed. It is recommended to not retreat with the same soil-residual herbicide(s) that caused the injury. Monitor fields and follow up with postemergence treatments if needed.

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