The warmer air temperatures forecast for the Corn Belt in the next week will undoubtedly accelerate both the growth of corn and those of weeds, leading to more fields being sprayed.

Aaron Hager, University of Illinois weed specialist, says it's important to remember that most post-emergence corn herbicides have a maximum stage beyond which broadcast applications should not be made. (See detailed chart of herbicides and application restrictions.)

These restrictions are usually indicated as a particular plant height or leaf stage, Hager says, and sometimes both of these are listed. For product labels that indicate a specific corn height and growth state, he recommends following the more restrictive of the two.

Application restrictions exist for several reasons, Hager says, but of particular importance is the increased likelihood of crop injury if applications are made beyond a specified growth stage.

"For example, some translocated herbicides should not be applied past the V6 stage because of an increased potential to cause adverse effects on the corn ear, which is being formed within the plant at about this stage," Hager says. "Other examples of growth stage restrictions have less to do with increased potential for corn injury, such as the 12-inch height restriction for post-emergence applications of atrazine."

While corn plant height is a very commonly used measure of growth stage, Hager says it may not accurately indicate a plant's true physiological maturity.

"Many agronomists agree that leaf number is a more accurate indicator of corn developmental stage," he says. "The primary techniques are counting leaves and counting leaf collars. Leaf counting begins with the short first leaf — the one with a rounded tip — and ends with the leaf that is at least 40% to 50% emerged from the whorl.

"Counting leaf collars also begins with the short first leaf, but it includes only leaves with a visible collar, or the light-colored band where the leaf joins the stem. Leaves in the whorl or those without a fully developed collar are not counted. The leaf-collar method often stages a corn plant at one leaf less than the leaf-counting method."

Because of the potential influence of adverse environmental conditions, Hager says you should be sure to accurately assess plant developmental stage by counting leaves or collars in addition to measuring plant height before applying any post-emergence herbicide. When counting leaves or leaf collars, be sure to account for leaves that might have been lost after frost or hail.

Hager encourages no-tillers to remember that some post-emergence corn herbicides also have application timing restrictions based on minimum corn size. For example, the Status label indicates that broadcast applications should be made when corn height is between 4 inches (V2) and 36 inches (V10), he says.