Key Takeaways

• Know the stage: Check corn growth stage before applying post-emergence herbicides.

• Read the label: Follow the most restrictive application requirement when using tank mixes.

• Protect the crop: Taller corn is more vulnerable to herbicide injury, especially when spray enters the whorl.

• Avoid stress: Apply herbicides only when both corn and weeds are actively growing and not under stress.

• Confirm compatibility: Verify the hybrid trait package matches the herbicide before spraying.

Effective early-season weed control is critical for minimizing crop–weed competition and preventing corn yield losses. In addition, excessive rainfall may reduce the effectiveness of pre-emergence herbicides by leaching or washing them off, thereby shortening the duration of weed control. 

When adequate soil moisture and warm temperatures favor weed germination and growth, corn fields can quickly develop dense weed populations (Figure 1), making timely post-emergence herbicide applications essential. Post-emergence herbicides are a key component of corn weed management programs. Several factors should be considered to maximize efficacy and minimize crop injury (Figure 2), including corn growth stage, weed species composition, and the relative heights of both crop and weeds.

As corn plants grow taller, they become increasingly sensitive to herbicide injury — particularly when spray nozzles deliver herbicide directly into the plant whorl. Herbicide labels specify the maximum corn growth stage beyond which broadcast applications are not recommended (see Table 1), and these restrictions should be followed closely to reduce the risk of crop damage.

How to Determine Corn Growth Stage

• Post-emergence herbicide application restrictions in corn are based on the growth stage specified on product labels. Growth stage limitations are typically expressed as a maximum corn leaf stage, plant height, or both. When both criteria are provided, always follow the most restrictive limit.
• When mixing two or more herbicides in the tank, adhere to the most restrictive label requirements among all products included in the mix.
• Corn height is measured from the soil surface to the arch of the uppermost leaf that is more than 50% emerged.
• Corn leaf stage is determined by counting visible leaf collars. A collar appears as a distinct band at the base of the leaf where it attaches to the stem. Leaves still emerging from the whorl do not have a visible collar and should not be counted.
• Leaf stages are designated using a “V” (vegetative) system: The first true leaf, a short and rounded leaf near the soil surface, is counted as V1. Each additional visible leaf collar corresponds to the next stage (V2, V3, V4, etc.). Corn development progresses through these vegetative stages until approximately V18, just prior to tasseling.

The Bottom Line

• Follow the corn growth stage restrictions on the herbicide label. Applications made beyond the growth stage may result in crop injury.
• Environmental conditions influence the absorption of post-emergence herbicides and potential for crop injury. Apply herbicides only when both the crop and weeds are not under stress.
• Herbicide product labels should be followed regarding spray additives that are recommended for improved weed control.
• Do not apply post-emergence herbicides with foliar fertilizers.
• It is important to clean the spray tank to avoid potential crop injury due to spray tank contamination.
• Drop nozzles can be used in taller corn for better weed coverage and to avoid spraying directly into the whorl.
• If post-emergence herbicides are applied beyond the labeled growth stage, corn ear development can be affected.
• Before applying herbicide, verify that the hybrid’s trait package is compatible with the product.

For more information on the efficacy of the herbicides in Table 1, refer to Nebraska Extension NebGuide EC130, “Guide for Weed, Disease, and Insect Management in Nebraska”.