University of Illinois

No. 10/May 29, 1998

Factors Contributing to the Likelihood of Corn Injury

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 combine to produce 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 and may exhibit a great deal of injury following a postemergence application. Producers who are concerned about a hybrid being sensitive to a particular herbicide should 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 levels may enhance uptake of soil-applied herbicides. Apart from enhancing herbicide uptake, environment-induced crop stress can often intensify crop injury from herbicides. The excessive soil moisture in many areas of Illinois is a good example of environmental stress.

Why is a crop under stress more likely to be injured from a selective herbicide? In most 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 corn is growing under favorable conditions, it will generally metabolize the herbicide well before the corn is injured enough to exhibit 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 enough for the herbicide to cause sufficient injury for symptoms to be manifested.

The herbicide itself can also impact crop response. For example, spray adjuvants applied with a postemer-gence herbicide can often enhance crop response. Most growth-regulator herbicides should be applied before corn reaches 8 inches in height or exhibits 5 leaves, whichever comes first. Broadcast applications of growth-regulator herbicides to corn larger than 8 inches or having more than 5 leaves greatly increase the probability of corn injury. A large percentage of the corn crop in Illinois is rapidly approaching these growth-stage limits, so the window for broadcast applications of these types of herbicides is rapidly closing. Contact postemergence herbicides, often applied with either crop-oil concentrate, a nitrogen fertilizer source (UAN, AMS), or both, can cause leaf speckling or burning.

Aaron Hager ( and Marshal McGlamery (, Department of Crop Sciences, (217)333-4424