Mark VanGessel, Extension Weed Specialist; email@example.com
Corn herbicides need to be applied at the correct timing to avoid crop injury; and the weeds need to be small (3-4 inches depending on the herbicide) for effective control. Many labels state from corn emergence to a certain size corn. However, some herbicides require the crop to be at a certain size before the herbicide can be applied (i.e. Status require corn to be at least 4 inches tall or V-2 stage). Almost all herbicides have a maximum crop size and this can range from V-2 to V-8. Maximum size depends on the herbicide and can vary based on whether the herbicide is applied over the top of corn or with drop nozzles. Applications after this time can result in crop injury and possibly yield reduction. Some labels refer to crop size based on height of corn in inches, collar stage, or leaf stage. Refer to the herbicide label to ensure applications are made at the appropriate crop stage. When corn height and collar number are given for the same herbicide, base decision on whichever feature is first attained. If tankmixing, use the guidelines based on the most restrictive herbicide.
Also, this year corn within the same field maybe at different stages. Evaluate the field and select the herbicide that allows application to the stages of corn that represent your field.