Diagnose Herbicide Injury To Corn

Farmers this week are asking about corn injury from herbicides. Symptoms have shown up in some fields in Iowa, in more fields in Illinois.

Published on: May 25, 2012

The number of observations and inquiries related to corn injury from herbicides has increased this past week. There are some fields in Iowa with injury and even more in Illinois, according to Extension weed management specialists at Iowa State University and the University of Illinois.

Instances of corn injury appear to be fairly widespread across Illinois. Direct application of postemergence herbicides and persistence of herbicides applied last season appear to be responsible for much of the injury. "Many postemergence corn herbicides routinely cause some corn injury," says Aaron Hager, University of Illinois Extension weed scientist. "Some growers, however, have the impression that this year there is a bit more injury than usual."

Diagnose Herbicide Injury To Corn
Diagnose Herbicide Injury To Corn

Hager and Iowa State University weed scientist Bob Hartzler say several factors can affect the sensitivity of corn to injury from postemergence herbicides, including:

* Hybrid: Corn hybrids have varying sensitivity to herbicides labeled for use in corn. For example, some corn hybrids are sensitive to specific ALS-inhibiting herbicides and tend to exhibit considerable injury following herbicide application. Many corn herbicide labels (especially labels of postemergence corn herbicides) carry warnings that certain corn hybrids could be sensitive to the active ingredient.

* Environmental conditions: High air temperatures and relative humidity levels favor rapid absorption of foliar-applied herbicides, but some environmental conditions can induce crop stress, slowing the rate at which the crops metabolize the herbicide and leading to increased herbicide-related injury. For example, cool air temperatures and wet soil can induce crop stress.

* Spray additives: Crop response may be enhanced when spray additives are applied with a postemergence herbicide or tankmix combination because of the increased rate of herbicide absorption into the plant. Be sure to read all label suggestions and precautions related to spray additives that should be either included or avoided when applying herbicides postemergence.

* Contamination: Herbicide residues in sprayer tanks and hoses and other equipment from prior applications may be applied inadvertently with the postemergence corn herbicide. These residues, either alone or in combination with the postemergence corn herbicide, may enhance the amount of corn injury. The type of contaminant and dose at which it is applied affect the severity of the corn response to spray contamination.

Injury problems from soil-applied herbicides also showing up in some fields

* Carryover: Soil residues of herbicides applied during the previous growing season (carryover of herbicide) appear to be another source of corn injury. Herbicide persistence in the soil, and the rate of chemical or microbial degradation, are influenced by many factors, including the specific chemistry of the herbicide and edaphic factors.

Soil pH is a critical factor affecting the persistence and degradation of many herbicides, including sulfonylurea and triazine herbicides, because it affects the rate of chemical degradation via hydrolysis. High soil pH (pH 7.0 or more) may slow the dissipation of certain herbicides by reducing the hydrolysis rate. Even when soil moisture is adequate, triazine and sulfonylurea herbicides degrade more slowly in soils that have high pH.

Soil moisture also influences the efficacy and persistence of soil-residual herbicides. Populations and activity of soil microorganisms that are responsible for microbial degradation of herbicides are greatly reduced when soil moisture is limited. Moreover, more herbicide is adsorbed to soil colloids in dry soils, making it unavailable for plant uptake and degradation by soil microbial populations.

"The very dry soil conditions encountered during parts of the 2011 growing season likely slowed the degradation of some soil-residual herbicides, particularly those degraded by soil microorganisms," says Hager. "Dry soil conditions early in 2012 allowed for unusually early corn planting, which in some instances occurred before the end of the rotational interval of the herbicide used in 2011.

Pay attention to herbicide label recommendations, such as crop rotation interval

Hager says there have been several reports of corn injury that appear to be caused by fomesafen carryover in Illinois. Veinal chlorosis or necrosis, which causes a striping effect on the leaves, is the most common of these symptoms. "In some instances, leaf veins almost appear clear or transparent, but the root system of affected plants usually shows no symptoms unless the foliar symptoms are extremely severe," he notes.

Because of the fomesafen's soil persistence and corn's sensitivity to fomesafen residues, the labels of most products that contain it, including Flexstar, Flextar GT and Prefix, indicate a 10-month rotational interval for corn.