No. 7 Article 9/May 12, 2006

Staging Corn Plants and Implications Associated with Herbicide Applications

Be sure to keep in mind when planning postemergence herbicide applications that maximum corn size is extremely critical. Quite often, if these restrictions are not followed, crop injury can occur, which increases the potential for subsequent crop yield loss. Herbicide labels often refer to plant height, crop growth stage (leaves or collars), or both when discussing corn growth stage limits for the application of postemergence herbicides. For products whose label indicates crop height and growth stage, it is important to follow the more restrictive of the two for each particular field. When environmental conditions include prolonged periods of cool air temperatures, corn usually remains relatively small with respect to plant height; however, it continues to advance physiologically. Shorter-season hybrids tend to produce shorter plants, and this should also be taken into consideration. Application restrictions based on corn developmental stage are usually stated with respect to the number of leaf collars present on the plants. Table 6 provides information describing maximum stage of corn growth for broadcast and directed applications of many corn herbicides.

As an example, the Clarity label indicates that 1 pint per acre can be applied to corn up to the five-leaf stage or 8 inches tall, whichever is more restrictive. Under cooler growing conditions, a corn plant may be less than 8 inches tall but have five or six leaves. If the herbicide application was made by looking only at corn height, there is a possibility that corn injury could occur because the application was made to corn beyond the labeled growth stage. Following the more restrictive of the two restrictions is extremely critical.

Since it is important to know the height and growth stage for accurate postemergence herbicide applications, here is a review of some common methods for determining these factors.

Corn height. Because corn height varies a great deal due to growing conditions and planting date, it's not a very accurate way to stage corn plants, but it is quick and universally understood. The normal method for determining corn height is to measure from the soil surface to the highest point of the arch of the uppermost leaf that is more than 50% emerged. Don't measure to the "highest point" on the plant, which is often the tip of the next emerging leaf above. Measure a number of plants and take the average.

Corn plant height method (note the uppermost leaf that the measurement is made to). This corn plant is staged at 5.5 inches tall.

Corn leaf method. The corn leaf method predates the leaf collar method (following) and is commonly used by hail adjusters and others. This method counts the number of leaves, starting from the lowest one (with a rounded tip) up to the last leaf that is more than 50% emerged. The "50% emerged" is a little subjective and is usually taken to be the leaf that has emerged enough so that its tip is starting to point down, below the horizontal. Do not count leaves younger (inside) than this one, even though you can see them in the whorl. This also should be done on a number of plants and averaged, or base the estimate on the stage that most of the plants are in.

Corn plant leaf number method (arrow refers to the last leaf counted). This corn plant is staged at five-leaf stage.

Corn collar method (V-stage). The leaf collar method is generally the easiest to use. It also relates better to physiological stage of the plant and thus to the effects of herbicides and the like. Since heights of plants at the same leaf stage vary widely depending on temperatures and moisture, leaf collar staging is more appropriate in most cases. Staging by leaf collar is done by counting the number of leaves with visible collars. The collar is the part of the leaf that joins the leaf blade and leaf sheath. Collars are not visible until the leaves are developed enough to emerge from the whorl. This means that the corn leaf method, which calls a leaf a leaf before its collar is visible, is one or two leaf stages ahead of the stage determined by the leaf collar method.

Corn collars as indicated by arrows.

Corn collar method (V-stage) as indicated by the number of arrows. This corn plant is staged at four leaves or V4.

There are variations in how different people apply these methods, including some who have reached the rather odd conclusion that the lowermost leaf is not a "real" leaf and so begin counting with the second leaf. Now, this leaf starts to deteriorate fairly early and so is often missing completely once plants have six or seven collars. But it's still a leaf in terms of estimating plant stage. One way to avoid having to guess how many lower leaves have been lost is to use scissors to cut the tip off the fifth or sixth leaf (remember which) so that you can start with that leaf in later visits.

Adverse weather such as hail or frost can damage corn leaves and make plants difficult to stage. Wind also causes considerable wear and tear on young leaves in many fields. Instead of guessing how many leaves might be missing on older plants, split the corn plant lengthwise down the stem to stage the plant. Splitting the stem will expose the nodes, future leaves, and (after about the six-leaf stage) the tiny, developing tassel at the tip of the stem. Look for an area of internode elongation, indicated by separation of the darker bands that are the nodes of the stem. The internodes are lighter in color, and the first elongated internode is typically just below the fifth leaf node on the plant. This internode is approximately 1 centimeter (0.4 inch) in length. The node above it can be used as a reference point for counting leaf collars on up the plant.

For a quick reference on staging corn, the Pocket Guide to Crop Development: Illustrated Growth Timelines for Corn, Sorghum, Soybean, and Wheat (C1389) is available. Or for more detailed information on staging corn growth, refer to the Iowa State University publication How a Corn Plant Develops.--Dawn Nordby, Aaron Hager, and Emerson Nafziger

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