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Couldn't apply pre-emerge herbicide? Try post- now

Miss your pre-emergence herbicide window? You can make it up with post-emerge, but you must do so with care to avoid even more yield loss and crop injury, according to one Illinois agronomist.

Keep a sharp eye on your crop to gauge the need for post-emerge herbicide, according to University of Illinois Extension weed specialist Aaron Hager. Doing so can give you an accurate look at the crop's development and, in turn, its need for herbicide. And, just because the ground may have needed a pre-emerge treatment before planting doesn't mean the same is true this late in the game.

"Reports of crop injury following the application of post-emergence herbicides have been common thus far in the season. The adverse growing conditions encountered soon after corn emergence persisted into the beginning of the post-emergence herbicide application window," Hager says. "Saturated soils can place the corn crop under additional stress. Crop injury is often more common when post-emergence herbicide applications are made to plants under stress."

Finding the fields where no pre-emergence treatments were made is usually easy; determining whether or not they need a post- treatment usually isn't, according to Hager.

"Be sure to scout cornfields before applying postemergence herbicides to accurately determine the crop's growth stage," he adds. "Adverse environmental conditions, such as prolonged periods of cool air temperatures or wet soils, can result in corn plants that are physiologically older than their height would suggest, so be sure to accurately assess plant developmental stage in addition to plant height."

Use both weed densities and projected economic returns to gauge whether a post-emerge treatment will pay off, even if a herbicide application wasn't part of your original plan. Hager suggests using a six- to 10% potential loss figure for weeds that, as of late June, are four to six inches tall or larger. It's at this point where the fields should be "treated as soon as possible to avoid additional crop yield potential.

"Using very simple math and an assumption of a 150-bushel corn crop, a 10% loss of yield potential equates to 15 bushels; that value multiplied by $6 a bushel comes out to $90 per acre loss because weeds were allowed to compete too long with the crop," Hager says. "If the plan at the beginning of the season was to save money by reducing or eliminating a pre-emergence herbicide, the amount of money seemingly saved up front might have been far exceeded by the loss of yield potential."

Finally, if you do apply a post-emergence herbicide application, make sure to get a solid grasp of your crop's life cycle and read product labels closely to avoid causing more harm than good.

"Pay close attention to the maximum corn stage listed on the respective herbicide label, and do not apply the product if corn exceeds the labeled stage," Hager says. "If you are tank-mixing two or more products, follow the most restrictive corn growth stage listed on any of the tank-mix component labels."

Miss your pre-emergence herbicide window? You can make it up with post-emerge, but you must do so with care to avoid even more yield loss and crop injury, according to one Illinois agronomist.

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