Issue No. 6, Article 4/April 29, 2005
Factors That Increase the Likelihood of Corn Injury
Several factors contribute to the possibility that a corn crop will exhibit injury symptoms following a herbicide application. In many cases, the cause is relatively clear, but in other instances, several factors contribute to the observed injury. If the cause is clear, the explanation also can be clear, but if several factors contribute (i.e., interact), 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 a herbicide application. If you are concerned about a hybrid being sensitive to a particular herbicide or herbicide family, contact the seed representative for information on the hybrid's expected response.
The environment has a large influence on the severity of injury symptoms from both soil-applied and postemergence herbicides. High temperatures and relative humidity levels favor enhanced absorption of postemergence herbicides. Adequate soil moisture 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 cool, wet soil conditions in some areas of Illinois are a good example of a stress induced by the environment.
Why is a crop under stress more likely to be injured by 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--the herbicide 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 injury, 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 5 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.
The recent period of cool air temperatures and wet soil conditions has undoubtedly induced some level of stress in emerged corn. This, coupled with overcast skies, will increase the likelihood of corn injury following application of certain postemergence herbicides. Unless weed pressure necessitates immediate action, producers may want to consider waiting a few days to let the corn recover from the stress of cold air temperatures before applying postemergence herbicides--Aaron Hager and Dawn Nordby