Several factors contribute to the likelihood that a corn crop will exhibit injury symptoms following a herbicide application. In many cases, the cause is relatively clear, but in many other instances, several factors contribute to the observed injury. If the cause is clear, the explanation can also be clear, but if several factors contribute to corn injury, fingers tend to be pointed in several directions and often little is resolved.|
Crop genetics can influence the degree of injury response. For example, certain corn hybrids are fairly sensitive to 2,4-D (or other herbicides for that matter) and may exhibit a great deal of injury following the herbicide application. If producers are concerned about a hybrid being sensitive to a particular herbicide or herbicide family, contact the seed representative for information on the hybrid's response to the herbicide or herbicide family in question.
The environment has a large influence on the severity of crop injury symptoms from either soil-applied or postemergence herbicides. High temperatures and relative humidity levels favor enhanced absorption of postemergence herbicides. Adequate soil moisture levels and low relative humidity can enhance uptake of soil-applied herbicides. Apart from enhancing herbicide uptake, environment-induced crop stress can often enhance crop injury from herbicides. The excessive soil moisture in many areas of Illinois is a good example of a stress induced by the environment. Why is a crop under stress more likely to be injured from a selective herbicide? In the majority of cases, herbicide selectivity arises from the crop's ability to metabolize (break down) the herbicide to a nonphytotoxic form before it causes much injury. For example, a grass herbicide used in corn cannot discriminate between giant foxtail and the corn crop--it attempts to control the corn just as it does the giant foxtail. When the corn is growing under favorable conditions, its ability to metabolize the herbicide generally occurs well before the corn is injured enough to express injury symptoms. If, however, the corn plant is under stress (which could be caused by a variety of factors), its ability to metabolize the herbicide may be slowed sufficiently to allow the herbicide to cause enough injury for symptoms to develop.
The herbicide itself can also determine the amount of crop response, and spray additives applied with a postemergence herbicide can often enhance crop response. Most growth regulator herbicides should be applied before corn reaches 8 inches in height or exhibits five leaves, whichever comes first. Broadcast applications of certain growth regulator herbicides to corn larger than these stages can greatly increase the probability of corn injury. Contact postemergence herbicides, often applied with either crop oil concentrate, a nitrogen fertilizer source (UAN, AMS), or both, can cause leaf speckling or burning. This type of injury can be greater when the corn crop is under stress from excess soil moisture.--Aaron Hager and Christy Sprague