Herbicides are increasingly applied postemergence (POST) in corn for season-long control of weeds. Fields should be scouted prior to POST herbicide applications to accurately determine the growth stage of corn and weeds. Herbicide product labels need to be followed according to the corn growth stage and other recommendations for POST applications. Environmental conditions should also be considered relating to how they may affect herbicide applications, efficacy, and crop safety.

When choosing a herbicide or tank mix combination to be applied POST, growers need to consider the weed species present, weed heights, and the corn growth stage. Taller corn is generally more sensitive to potential herbicide injury, particularly when nozzles are applying the spray directly into the whorl of the plant. Drop nozzles can be used in taller corn (generally 24 inches or more) to help provide better weed coverage and prevent spraying directly into the whorl.  

Herbicide product labels provide directions for over-the-top broadcast and directed drop nozzle applications in corn. Labels provide a maximum corn growth stage after which broadcast applications should not be made (Table 1). In corn with Roundup Ready® 2 Technology, Roundup® brand glyphosate-only agricultural herbicides can be applied broadcast up to the V8 growth stage or 30-inch tall corn, whichever comes first. Drop nozzles should be used for optimum spray coverage and weed control when corn is 24 to 30 inches tall. When corn is 30 to 48 inches tall, only use drop nozzles and avoid spraying into the whorls of the corn plants. 

Some labels also indicate the minimum corn growth stage before POST applications should be made. For example, Capreno® herbicide can be applied POST when corn reaches the V1 stage up through V7 or 20-inch tall corn, whichever comes first. Broadcast applications of Capreno herbicide in corn grown for seed are recommended from the V1 to V5 growth stages.   

Growth stages on herbicide product labels are usually indicated as a corn leaf stage or plant height, and sometimes both are listed. The growth stage that is most restrictive should be followed when both corn leaf stage and height are listed on the label. When using a tank mixture, follow the recommendations for the most restrictive label language of the products being used in the tank mix. 

Plant height is determined by measuring from the soil surface to the arch of the uppermost leaf that is more than 50% emerged (Figure 1). Plant height may not be an accurate determination of growth stage, because adverse environmental conditions can result in corn plants that are physiologically older than their height suggests.  

Corn leaf stage is determined by counting collars when leaves emerge from the whorl and a band is formed around the stem (Figure 1). Leaves that are just emerging from the whorl will not have a visible collar and are not counted. Leaf stages are designated using a “V” (vegetative) to represent each leaf during vegetative development. The first true leaf (coleoptile leaf) on corn is the short, rounded leaf near the soil surface, which is counted as V1. Each successive, visible leaf collar is counted as V2, V3, and all the way to V18, which emerges prior to tasseling. As corn plants grow, the lower leaves can die or tear away, making it difficult to accurately count the collars. Corn generally loses its coleoptile leaf by the time it reaches the V5 growth stage. To stage older plants, dig up the plant and split the stalk down into the root ball. Find the triangular “woody” base of the stalk and locate the first internode above the base. The woody, horizontal node is the point of attachment for the fifth leaf or collar. For example, if you can count 5 visible leaf collars above this point, the corn plant is in the V10 growth stage. 

• ALWAYS READ AND FOLLOW LABEL DIRECTIONS. The type and amount of corn injury from a POST application beyond the labeled window for the herbicide(s) can be dependent on the herbicide(s), the exact timing of application, and environmental stress.   
• Environmental conditions influence the absorption herbicides and potential crop tolerance. Corn under stress may not metabolize some herbicides quickly enough to avoid crop injury. Cool temperatures and wet soils can slow the growth of corn and alter its development. Warm and humid conditions promote rapid absorption, while cool and dry conditions may slow the crop’s development. Corn can become more responsive to herbicides during favorable growing conditions due to changes in the leaf cuticle that may allow greater absorption. 
• Spray additives can increase the rate of herbicide uptake by the crop. Injury symptoms resulting from spray additives include chlorotic mottling or necrosis of leaves and may only be temporary as the crop recovers. Herbicide product labels should be followed regarding spray additives that are recommended. 
• Herbicide residues from previous applications may remain in the spray tank causing contamination.  This contamination could cause an unwanted interaction with the herbicides applied to corn.   
• POST-applied herbicide Injury symptoms can include leaf chlorosis or necrosis, onion leafing, internode stacking, rat tailing, ear pinching, ear bottlenecking, brace root malformation, and green snap. Corn ear development can be affected if POST-applied herbicides are applied late in the growing season. Therefore, it is important to always read and follow the corn growth stage restrictions on the herbicide product labels. 

Sources:

Jhala, A. 2017. Consider corn growth stage when applying postemergence herbicides. University of Nebraska. CropWatch. https://cropwatch.unl.edu.

Hager, A. 2018. Corn growth stage and postemergence herbicides. University of Illinois. Pest Management Bulletin. http://bulletin.ipm.illinois.edu.

Bell, J. 2018. Corn growth stages and post emergent herbicide timing. Texas A&M University. AgriLife. https://agrilife.org.

Lingenfelter, D. 2019. Corn herbicide application timings and restrictions. Penn State University Extension. https://extension.psu.edu. 

Web sites verified 3/4/2020. 4012_S4