Herbicide application restrictions on herbicide labels exist for several reasons, but the most important is the fact that crop injury is more likely to occur if applications are made outside a specified growth stage or range.
Most postemergence corn herbicides can be applied at various crop growth stages, but almost all product labels indicate a maximum growth stage after which broadcast applications should not be made. Some labels also indicate the minimum growth stage before which applications should not be made. These growth stages are usually indicated as a particular plant height or leaf stage; sometimes both are listed.
For product labels that indicate a specific corn height and the growth stage, be sure to follow the more restrictive of the two. That's the advice of Extension weed control specialists at Iowa State University and the University of Illinois.
How do you measure the height of a corn plant? Is height a good indicator?
Herbicide labels tend to state the restrictions in terms of plant height, which may not always provide an accurate indication of the plant's true physiological maturity. Moreover, the final measure of height depends on the benchmark used.
Generally, corn plant height is the distance from the soil surface to the arch of the highest leaf that is at least 50% emerged from the whorl, says University of Illinois weed control specialist Aaron Hager. Several plants in a field should be measured and the numbers averaged.
The problem with using plant height to determine growth stage is that it is influenced by many factors, including genetics and the growing environment. Adverse environmental conditions such as cool air/soil temperatures and hail can greatly retard plant growth, resulting in corn plants that are physiologically older than their height suggests.
Counting number of leaves is a more accurate measure of corn growth stage
Because of this possibility, it is important to assess other indicators of plant developmental stage before applying any postemergence herbicide. Many agronomists agree that leaf number, counting either leaves or leaf collars, is a more accurate measurement of corn's developmental stage.
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. Leaf-collar counting also begins with the short first leaf but includes only leaves with a visible collar (the light-colored band where the leaf joins the stem). Leaves in the whorl or those without a fully developed collar are not counted. This method quite often stages a corn plant at one leaf less than the leaf-counting method. Pay careful attention to which method (leaf or collar number) is indicated on the herbicide label.
When counting leaves or leaf collars, be sure to account for leaves that might have been lost from the plant after a frost or hail storm.
"If you believe one or more corn leaves has been lost due to frost, it's advisable to err on the high side when estimating loss because potential for corn injury generally increases as plants become more mature," says Hager. "If a second postemergence application will be made later in the season, don't forget to include leaves that might have been lost earlier in the season in the count."