Herbicide considerations for replanted corn

Following recent and excessive precipitation, many corn producers are now scrambling to replant. While there are many agronomic considerations associated with replanting, University of Illinois weed scientist Aaron Hager said farmers should keep weed control/herbicide issues in mind.

“Herbicide-resistance traits in the replanted hybrids should be taken into account,” says Hager, an associate professor in the Department of Crop Sciences at U of I. “For example, if you initially planted a glyphosate-resistant corn hybrid and have areas that need to be replanted, you can replant with a similar glyphosate-resistant hybrid or choose to replant with one that’s not glyphosate-resistant. If you take the second option, you will have to take special precautions to reduce drift with any postemergence glyphosate application, as these plants will be extremely sensitive to glyphosate.”

Hager said farmers should consider the interval between the last herbicide application and corn replanting.

“For soil-applied corn herbicides, replanting can proceed whenever field conditions are feasible,” he said. “However, for some postemergence corn herbicides, there are intervals between application and replanting. If replanting a corn field previously treated with Spirit, for example, four weeks must elapse between the herbicide application and planting. For NorthStar, the interval is 14 days. For Permit or Yukon, you need to wait one month.”

While most soil-applied herbicides allow more than one application per season, a few, such as Acuron and Resicore, can be applied only once. In instances where small areas of a field will be replanted, Hager says some farmers may elect to simply replant without applying any additional residual herbicide.

“However, if you decide to make a second application of a particular corn herbicide, keep in mind that many product labels indicate a maximum per-acre rate that can be applied during one growing season,” he said.

If farmers need to control corn from the first planting, Hager recommends tillage as an effective first choice. Several herbicides can control existing corn plants if tillage isn’t an option, but Hager says careful attention must be given to what, if any, herbicide resistance trait(s) the existing corn plants contain.

“As you might imagine,” Hager said, “glyphosate is very effective for controlling existing stands of corn sensitive to glyphosate. Corn replanting can occur immediately after application, but control might be improved if at least 24 hours elapses between application and replanting. Glyphosate also would control sensitive weeds that might have emerged with the initial stand of corn. Be very cautious to avoid drift when spraying glyphosate, especially if spraying around wet holes.”

Other herbicides to control emerged corn include paraquat and glufosinate (only hybrids sensitive to glufosinate), although previous research with these herbicides has demonstrated that complete control is not always achieved. Performance of these products can be improved when applied in combination with atrazine or metribuzin. Paraquat and glufosinate would also control a broad spectrum of emerged weeds.

Corn hybrids resistant to glyphosate, glufosinate, or both can be controlled with Select Max prior to replanting field corn. According to label specifications, farmers should apply 6 fluid ounces per acre to control glyphosate-resistant field corn up to 12 inches tall.

“Applications should include NIS and AMS (do not use a COC or MSO in this particular use), and care must be taken to avoid in-field overlaps or excessive injury to the replanted corn might occur. Glyphosate can be tank-mixed with the Select Max to control emerged broadleaf weed species. Do not replant fields treated in this way sooner than six days after application or severe injury to the replanted corn can occur,” Hager said.

Product labels of ACCase-inhibitors including Poast, Poast Plus, Fusion, Fusilade, Select, and Assure II require an interval between application and rotation to or replanting with grass crops such as corn. These intervals range from 30 (Poast, Poast Plus, Select) to 60 (Fusion, Fusliade) to as many as 120 (Assure II) days, making these products unlikely choices for this particular use. Severe injury to replanted corn can occur if soil residues of ACCase-inhibiting herbicides are taken up by emerging corn plants.

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