Issue No. 8, Article 7/May 19, 2006
Corn Injury and Herbicides
Several factors contribute to the possibility that a corn crop will exhibit injury symptoms following herbicide application. Often the cause is relatively clear, but just as often several factors contribute to the observed injury. If the cause is clear, the explanation also may be clear, but if several factors contribute to corn injury (i.e., interact), fingers tend to point in several directions.
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 herbicide application. If you are concerned about a hybrid's 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. Speaking of 2,4-D, keep in mind that corn injury is possible following preplant or preemergence applications. When growth regulator herbicides are brought into close contact with a germinating corn seed or emerging corn seedling, the potential for crop injury is increased. Cool, wet conditions following 2,4-D application and before corn emergence tend to increase the potential for injury (such as enhanced elongation of the mesocotyl).
The environment greatly influences 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 recent cool air temperatures and wet soil conditions in some areas of Illinois are good examples of environment-induced stress.
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 attempts to control the giant foxtail. When the corn is growing under favorable conditions, it metabolizes the herbicide before excessive injury occurs. 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. Keep in mind that several nonherbicidal factors also can cause growth abnormalities in corn (leafing-out underground induced by cold or soil crust, for example).
The herbicide itself can also influence the amount of crop injury, and spray additives applied with a postemergence herbicide or tank-mix combinations may 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 (or even certain soil-applied herbicides used following corn emergence), often applied with either crop oil concentrate, a nitrogen fertilizer source (UAN, AMS), or both, can cause leaf speckling, burning, or bleaching. This type of injury can be greater when the corn crop is under stress from excess soil moisture or cool temperatures.
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 wait a few days to allow corn to recover from the stress of cool air temperatures before applying postemergence herbicides.--Aaron Hager