Issue No. 7, Article 3/May 21, 2010
Corn Growth Stages and Postemergence Herbicides
The Illinois corn crop currently exists at various stages of growth. Some fields have already been treated with a postemergence herbicide, while other stands will be replanted due to flooded field conditions. The warmer air temperatures forecast for the remainder of the week will undoubtedly accelerate weed growth, so expect that more fields soon will be sprayed. It's important to remember that most postemergence corn herbicides can be applied to corn at various growth stages, but almost all product labels indicate a maximum stage beyond which broadcast applications should not be made (Table 1). These restrictions 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 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. Other examples of growth stage restrictions have less to do with increased potential for corn injury, such as the 12-inch height restriction for postemergence applications of atrazine.
Corn plant height is a very commonly used measure of growth stage, but it may not accurately indicate a 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 influenced by many factors, including genetics and the growing environment. Adverse environmental conditions, including cool air or soil temperatures, hail, and frost, 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 indicator of corn developmental stage. 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 (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, be sure to accurately assess plant developmental stage by counting leaves or collars in addition to measuring plant height before applying any postemergence herbicide. When counting leaves or leaf collars, be sure to account for leaves that might have been lost after frost or hail.
Farmers are also encouraged to remember that some postemergence 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). Table 2 lists some of the postemergence corn herbicides that specify minimum corn size restrictions.--Aaron Hager