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University of Illinois associate professor of weed science Aaron Hager says that the below-freezing temperatures last week damaged some emerged corn in some parts of Illinois.
"While the damage appears not to be widespread, it is very likely that some growers will opt to replant damaged stands," he said. Losses of the initial corn stand could range from complete to partial, so farmers may need to control existing corn plants before replanting.
According to Hager, "Glyphosate is very effective for controlling existing stands of corn hybrids sensitive to glyphosate." Corn replanting can occur immediately after application, but control of existing corn plants could be improved if at least 24 hours elapse between application and replanting. Glyphosate also controls sensitive weeds that might have emerged with the initial stand of corn.
Other options to control emerged corn include paraquat and glufosinate (only for hybrids sensitive to glufosinate). However, "Previous research with these herbicides has demonstrated that complete control is not always achieved," notes Hager.
Performance is often enhanced when these products are applied in combination with atrazine or metribuzin. "Paraquat and glufosinate would also control a broad spectrum of emerged weeds," he said.
Hager said that glyphosate-resistant corn from a first planting can be controlled with Select Max prior to replanting. "The label says to apply six fluid ounces per acre for control of glyphosate-resistant field corn up to 12 inches tall," he said. Applications should include NIS and AMS—do not use a COC or MSO—and care must be taken to avoid in-field overlaps, or excessive injury to the replanted corn might occur.
Glyphosate can be tankmixed with the Select Max to control emerged broadleaf weed species. "DO NOT replant corn in fields treated with Select Max sooner than six days after application, or severe injury to the replanted corn can occur," warned Hager.
The product labels of Poast, Poast Plus, Fusion, Fusilade, Select, and Assure II specify an interval that must elapse between application and rotation to, or replanting with, grass crops such as corn. These intervals range from 30 days (Poast, Poast Plus, Select) to 60 days (Fusion, Fusilade), to as many as 120 days (Assure II), making these products unlikely choices for this particular use. Severe injury to replanted corn can occur if soil residues of any of the ACCase-inhibiting herbicides described above are taken up by the emerging corn plants (Figures 1 and 2).