Corn Height Impacts Postemergence Herbicide Options
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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.
Courtesy: University of Illinois Extension
As the corn crop moves into the late postemergence or rescue growth stages (V6 to V8), herbicide options diminish. On corn up to 20 inches tall or V5-V6, you can use nicosulfuron. Before corn is at the V8 growth stage, you can apply topramezone, pendimethalin, mesotrione, dicamba, S-metolachlor, tembotrione, and glyphosate.
Certain herbicides can be used on taller corn with drop nozzles to keep the spray solution out of the corn whorl. These products include primisulfuron, 2,4-D, bromoxynil, and fluthiacet. Apply before the tassel emerges, according to label instructions.
Courtesy: University of Nebraska-Lincoln
For herbicide-resistant corn hybrids, glyphosate use on glyphosate-resistant corn has a maximum corn height of 30 inches or V8. Glufosinate maxes out at 24 inches or V7 corn growth stage on glufosinate-resistant corn. “Apply Enlist One or Enlist Duo when weeds are small and Enlist corn is no larger than V8 growth stage or 30 inches tall, whichever occurs first," explains Jhala. "For corn heights 30 to 48 inches, apply only using ground application equipment using drop nozzles aligned to avoid spraying into the whorl of corn plants."
Read the Label
Be sure to check the label for all crop safety details. “Environmental conditions that stress the crops and weeds will influence the absorption of herbicides, spray additives and foliar fertilizers, so it’s important to read the label to reduce your risk,” he adds.
Agronomists, crop consultants and weed scientists all suggest checking with seed companies for specific hybrid-herbicide recommendations, as some genetics may have more--or less--tolerance to specific herbicide families. Check out Pioneer’s 2022 Corn Hybrid-Herbicide Management Guide.
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