With most of the Midwest’s corn acres planted, growers now grapple with decisions about weed control. The correct timing of herbicides applied after the crop emerges will help maintain top yields, especially with heavy weed growth expected from the wet and warm spring weather.

A general recommendation from weed scientists is to apply postemerge herbicides on corn before weeds are 2 in. tall, says Aaron Hager, University of Illinois Extension weed specialist. This timing will kill weeds early before crop yield loss results from weed interference.

Most postemerge herbicides may be applied at various crop growth stages. Product labels detail the maximum growth stage for application of the herbicide, after which an application is detrimental, Hager says. Plus, a few products state a minimum growth stage, before which the product should not be applied.

“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,” he adds.

Most labels use corn plant height to determine growth. Hager says this height is generally determined by measuring from the soil to the arch of the uppermost leaf that is at least 50% emerged from the whorl. He recommends measuring several plants in the field and averaging the numbers.

Some agronomists say leaf number is a more accurate measure of corn development. Leaf counting should start with the short first leaf and end with the leaf that is 50% emerged from the whorl. Counting leaf collars is another method, which starts with the first short leaf and includes only leaves with a visible collar.

For more information, read The Bulletin published by University of Illinois Extension.