The labels of most postemergence corn herbicides allow applications at various crop growth stages, but almost all product labels indicate a maximum growth stage beyond which broadcast applications should not be made, and a few even a state 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 of these are listed. For product labels that indicate a specific corn height and growth state, be sure to follow the more restrictive of the two. Application restrictions exist for several reasons, but of particular importance is the increased likelihood of crop injury if applications are made outside a specified growth stage or range.
Corn plant height is commonly used on many herbicide labels but plant height may not always provide an accurate indication of the plant’s true physiological maturity. Determining plant height may seem relatively straightforward, but using different benchmarks for measurement can lead to different plant heights. Generally, corn plant height is determined by measuring from the soil surface to the arch of the uppermost leaf that is at least 50% emerged from the whorl. Be sure to measure several plants in a given field and average the numbers. Plant height is obviously influenced by many factors, including genetics and the growing environment. Adverse environmental conditions, such as cool air/soil temperatures, hail, etc., can greatly retard plant height and result in corn plants that are physiologically older than their height suggests.
Many agronomists agree that leaf number is a more accurate measurement of corn developmental stage. Counting leaves and counting leaf collars are the two primary techniques used. Leaf counting begins with the short first leaf (the one with a rounded tip) and ends with the leaf that is at least 40–50% emerged from the whorl. Counting leaf collars 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. The leaf collar method quite often stages a corn plant at one leaf less than the leaf counting method.
Adverse environmental conditions, such as one or more frost events, can sometimes result in corn plants that are physiologically older than their height would suggest, so be sure to accurately assess plant developmental stage (i.e., leaf/collar number) in addition to plant height before applying any postemergence herbicide. 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 but are uncertain the actual number lost, it’s advisable to error on the high side (i.e., assume more leaves) than the low side since potential for corn injury generally increases as plants become more mature. 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.