While some areas of Illinois have struggled to plant corn between frequent rains, other areas have corn that is close to closing the rows. We wanted to remind producers that many postemergence corn-herbicide labels indicate a maximum corn size or development stage for broadcast applications. If these restrictions are not followed, the likelihood of corn injury may increase dramatically.
When application restrictions are found on the labels of postemergence herbicides, they are generally stated with respect to corn height, developmental stage, or both. While corn height appears self-explanatory, some ambiguity often exists with respect to where on the corn plant you measure to. In general, many people determine corn plant height by measuring from the soil surface to the top of the whorl or to the arch of the uppermost leaf that is more than 50% emerged. Restrictions based on corn developmental stage are usually stated with respect to the number of leaf collars present on the plants. Many agree that the leaf-collar method is a more accurate method to determine physiological plant age than is plant height. In 2002, several cases of severe corn injury occurred when some postemergence herbicides were applied to corn shorter than the maximum height listed on the product labels but having more leaf collars than would be expected for corn that height. The take-home message is to follow the most restrictive corn stage (height or leaf collar) listed on the product label. Table 3 summarizes the recommended timings for several postemergence corn herbicides. Always check the product labels for specific directions and use limitations.
During the weekend of May 30, nighttime low temperatures dropped into the mid- to upper 30s in some areas of Illinois. Within a few days, several corn plant samples sent to the Plant Clinic showed leaf damage caused by these low temperatures. Any cornfields that were treated within a few days before or after these cold temperatures should be scouted carefully, as herbicide injury may be accentuated.--Aaron Hager and Christy Sprague