Post-Emergence Herbicides in Corn

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It is time for farmers to control weeds in their corn fields. However, the cool, wet start to the growing season makes it doubly important to read and follow herbicide labels.

The post-emergence herbicide labels they’ll be following usually allow applications to be made at various growth stages says University of Illinois Extension Weed Scientist Aaron Hager. He says it is really important to read the label, making sure to get the height, or the stage, maybe both, of the crop correct.

This is because most all of the products for corn have a growth stage listed on the label beyond which applications, at least broadcast applications, should not be made. It is usually either plant height - measured at the highest arch of the uppermost leaf at least 50% out of the whorl - or a leaf number. Hager says if both are listed, then it is important to use the more restrictive of the two, For example, because of some of the weather conditions we’ve had across a large part of the state this year we may have corn plants which are older than their height would suggest. Using the leaf collar method is typically a better way to stage the development of the corn plant. If you can do both the height and the counting, the leaf collar method is the better method to determine the stage of the corn plant."

Using the leaf collar method is typically a better way to stage the development of the corn plant. - Aaron Hager, University of Illinois

Corn plants under stress conditions may be more prone to injury from post-emergence herbicides. On that note, Hager says farmers should be sure to consult the product label when selecting spray additives. Many labels suggest changing from one type of additive to another when the corn crop is stressed. Also, trying to save a trip across the field by applying a post-emergence corn herbicide with liquid nitrogen as the carrier is not advisable. The U of I weed scientist says while applying high rates of UAN by itself can cause corn injury, adding a post-emergence herbicide can make it worse.