Corn Height Impacts Postemergence Herbicide Options

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Aerial view of green tractor sprayer in field

Optimal postemergence weed control in corn requires timely application. But if treatment is delayed due to weather, put on your scouting shoes. Corn sensitivity to herbicides generally increases as the plant gets taller, especially if herbicides enter the whorl. Knowing the crop's growth stage will help determine which herbicides to use – and avoid – to prevent crop injury and potential yield loss.  

“Check each herbicide label to determine if there are application restrictions by the corn growth stage. If applying a tank-mix, it’s best to use the growth stage of the product that is most restrictive,” says Amit Jhala, University of Nebraska Extension weed management specialist.  

Calculate the Collars

Courtesy: Iowa State University

Count the collars formed as leaves emerge from the whorl and create a band around the stem. For example, the V8 growth stage (V equals vegetative, 8 is the number of collars from the ground up) usually occurs when the corn plant is approximately two feet tall. Don’t count newly emerged leaves until a visible collar is formed. For herbicide labels that list corn height, select an assortment of plant sizes in a field to get an average, measuring from the soil surface to the arch of the top leaf that is more than 50% emerged. 

On non Enlistâ corn, you can broadcast 2,4-D up to a maximum corn height of 8 inches. Other herbicides—used alone, in a pre-mix or tank mix—also max out on shorter corn. Atrazine, for example, should not be applied on corn over 12-inches tall.