Due to early-season conditions, the Illinois corn crop is at several different stages, with corn sizes ranging from emergence to growth stages critical to postemergence herbicide applications. Issue no. 6 of the Bulletin addressed the use of soil-applied herbicides after corn emergence to control early-emerging weeds and the critical timing associated with this practice. However, this window has closed in many areas of the state, and we need to look at postemergence herbicides to control later-emerging grass and broadleaf weeds. One thing to keep in mind when planning these postemergence herbicide programs and rescue applications is that maximum corn size is extremely critical. Many times, if these restrictions are not followed, substantial crop injury can result that can lead to yield reductions.|
Herbicide labels often refer to plant height, crop growth stage, or both when discussing timing of postemer-gence applications. On labels where both crop height and growth stage are mentioned, it is important to follow the more restrictive of the two. This is extremely critical with the cool weather that the corn crop has been experiencing this season. During these conditions corn usually remains relatively small in regards to plant height. However, corn continues to advance developmentally. For 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 only looking 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 timely postemergence herbicide applications, here are a couple of methods for determining these factors. The most common method for determining corn height is done by using free-standing plants. When measuring corn height, measure from the soil surface to the arch of the uppermost leaf that is more than 50% emerged. This should be done on a number of plants and then averaged to overcome any variability among corn plants in the field. In determining corn growth stage, the collar method is the most appropriate to use. Vegetative growth can be 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 apparent until the leaves are developed and emerged from the whorl. Therefore, leaves that are just emerging from the whorl are not counted. For example, a plant may appear to have six leaves. However, after closer examination, there may only be four leaves with collars. Therefore, this plant would be considered in the V4 growth stage. For more information on staging corn growth, refer to Iowa State publication "How a Corn Plant Develops" at http://maize.agron.iastate.edu/corngrows.html#how.
Table 3 summarizes the recommended timings for several postemergence herbicides used in corn. Always check the product label for specific directions.--Christy Sprague and Aaron Hager