The 2017 Illinois corn crop currently is at various stages of development. Applications of postemergence corn herbicides continue to be made across areas of Illinois, although the recent precipitation has delayed applications in some areas. Even though applications may be delayed, adequate soil moisture coupled with warm temperatures will certainly promote rapid growth of emerged weeds.
Properly timing the application of the postemergence herbicide is critical toward achieving the goal of removing weed interference from the corn crop before the weeds adversely impact (i.e., reduce) corn grain yield. Unfortunately, it’s not possible to accurately predict the specific day after planting or emergence when weed interference begins to reduce corn yield. This interval is influenced by many factors and can vary based upon the weed spectrum, the density of certain species, available soil moisture, etc. Weed scientists generally suggest an interval, based either on weed size (in inches) or days after crop/weed emergence, during which postemergence herbicides should be applied to avoid crop yield loss via weed interference. In corn, it is often recommended to remove weeds before they exceed 2 inches tall. The longer weeds are allowed to remain with the crop the greater the likelihood of crop yield loss.
It’s important to remember that 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.
As mentioned, 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.
Corn plants under stress conditions may be more prone to injury from postemergence herbicides. Stress can arise from a number of factors, including cool temperatures and wet soils. Be sure to consult the product label when selecting spray additives to include with postemergence herbicides. Many labels suggest changing from one type of additive to another type when the corn crop is under stressful growing conditions. Attempting to save a trip across the field by applying a postemergence corn herbicide with a liquid nitrogen fertilizer solution (such as 28% UAN) as the carrier is not advisable. While applying high rates of UAN by itself can cause corn injury, adding a postemergence herbicide can greatly increase corn injury.
Labels of several postemergence corn herbicides (most commonly ALS-inhibiting herbicides but also some HPPD-inhibiting herbicides) include restrictions with respect to applying the product to corn previously treated with certain soil insecticides. Be sure to consult the respective herbicide label for other restrictions and limitations.