No. 8 Article 8/May 18, 2007

Corn Injury and Herbicides

Corn response (i.e., injury) caused by herbicides applied for in-crop weed control can range from no visible response to near-complete crop loss. Determining the reason(s) for any observed crop injury can be challenging, because several interacting factors may contribute to the severity of response. If the cause is readily discernible, the explanation and prognosis also may be straightforward, but if multiple factors contribute to crop injury, the process of assessment and prognosis may become less precise.

Crop genetics can influence the degree of injury response. Certain corn hybrids, for example, are sensitive to 2,4-D (or other herbicides for that matter) and may exhibit a great deal of injury following herbicide application. The labels of many corn herbicides, especially postemergence herbicides, have precautionary statements about the potential for certain corn hybrids to be more sensitive than others to the particular active ingredient. If you are concerned that a particular hybrid may be sensitive to a particular herbicide or herbicide family, contact the seed company representative for information.

If more than one formulation of a particular active ingredient is commercially available, the choice of which formulation to use, especially for postemergence applications, also can influence the occurrence of corn injury. As described in last week's issue of the Bulletin (no. 7, May 11, 2007), ester formulations of 2,4-D tend to be absorbed through the leaf surface faster than amine formulations. Applying 2,4-D esters postemergence with additives such as COC, or tankmixing herbicides with formulations that can "behave" similarly to a spray additive, can increase the rate of 2,4-D uptake into the corn, potentially leading to enhanced injury.

The environment has a large influence on the severity of crop injury symptoms from both soil-applied and postemergence herbicides. High air 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. Rapid herbicide absorption into the corn plant may temporarily overwhelm the plant's ability to break down the herbicide, leading to injury symptoms.

Apart from enhancing herbicide uptake, environment-induced crop stress can enhance crop injury from herbicides. Cool air temperatures and wet soil conditions are good examples of environmental conditions that can induce 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-control herbicide used in corn cannot discriminate between giant foxtail and a corn plant; 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 rapidly 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 enough that injury symptoms develop.

The herbicide itself can influence the severity of crop injury, and spray additives applied with a postemergence herbicide or tankmix combinations may enhance crop response. Be sure to read all label suggestions and precautions related to spray additives that should be either included or avoided when applying herbicides postemergence. This suggestion includes postemergence applications of traditionally soil-applied herbicides, described in a previous article ("Postemergence Applications of 'Soil-Applied' Corn Herbicides," no. 6, May 4, 2007).--Aaron Hager

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