Three critical tips when applying postemergence corn herbicide

In some parts of the Corn Belt, the plant is already towering to two feet tall. This has much of the crop in the northern region far enough along to be in V3 growth stage. As early as corn is growing, farmers need to remember these three strategic tips to remember when applying their postemergence corn herbicide.

Size is significant

All postemergence corn herbicide labels spell out the usage at different growth stages of the crop, typically by the height of the crop or by leaf stage. Aaron Hager, Associate Professor of Weed Science at the University of Illinois, recommends following the most restrictive usage instructions to avoid crop injury.

"Corn plant height is commonly used on many herbicide labels, but height is not always an accurate indicator of a plant's true physiological maturity. Determining plant height may seem relatively straightforward, but using different benchmarks for measurement can lead to different plant heights," said Hager. "Generally, height of a corn plant is determined by measuring from the soil surface to the arch of the uppermost leaf that is at least 50% emerged from the whorl (a circle of three or more leaves at the same joint of the stem). Be sure to measure several plants in a given field and average the numbers. plant height is obviously influenced by many factors, including genetics and the growing environment. Adverse environmental conditions, such as cool air or soil temperatures, hail, and others, can greatly retard plant height and result in corn plants that are physiologically older than their height suggests."

Hager says farmers should accurately count leaves on the crop and determine the crop's height before applying. To count leaves, start with the short first leaf and end with the leaf at least half emerged from the whorl. Remember that some leaves may have been lost from hail or frost.

Critical timing creates better yields

Applying postemergence herbicide at the proper stage of growth, as we stated above, can make or break a corn crop's full yield potential. The longer weeds sit in the field among the crop, the greater the risk of a reduced yield. Weeds sitting in a field are more likely to absorb moisture necessary to the crop's development. In addition, the weed's root can damage the corn's root, causing the plant to become "floppy" and inadequate to continue proper growth.

"Weed scientists generally suggest an interval, based on either weed size (in inches) or days after crop/weed emergence, during which postemergence herbicides should be applied to avoid crop yield loss through weed interference," says Hager. "A sommon recommendation is to remove weeds in corn before they exceed about two inches tall."

Smaller weeds are easier to control than larger ones. In addition, Hager states it's been proven that herbicide absorption is more effective in younger weeds than older ones.

Apply when conditions are right

Mother Nature plays such a vital role with everyday duties around the farm. The same can be said about applying postemergence herbicides to corn.

"High air temperatures and relative humidity levels favor rapid absorption of postemergence herbicides, and environmental-induced crop stress can enhance injury from herbicides. Cool air temperatures and wet soil are examples of environmental conditions that can induce crop stress. Crops under stress often cannot metabolize herbicide rapidly enough to preclude injury symptoms," says Hager.