Farmer interest in planting cover crops has increased tremendously in the last few years. Cover crops can provide a number of benefits to a corn-soybean cropping system. The use of residual herbicides has increased at the same time, and the potential impact these herbicides may have on establishing cover crops is an important consideration.

COVER CROP DAMAGE: Will a previously applied residual herbicide affect the cover crop you seed in a field in late summer or in fall? It depends on the kind of herbicide and also on the type of cover crop.

What is the effect of various residual herbicides on establishment of a cover crop? It depends on the type of herbicide that's been applied to the field and also on the type of cover crop you are planting. Iowa State University Extension weed management specialist Bob Hartzler and ISU Extension field agronomist Meaghan Anderson have looked into this question. They've conducted research testing residual herbicides and cover crop species, and provide the following information and observations. Herbicide labels usually include information regarding restrictions for rotational crops, and in many situations the restrictions were not developed in respect to cover crops.

Hartzler says the two primary reasons for the rotational crop restrictions are: 1) herbicide residues interfering with crop establishment, and 2) residues accumulating in rotational crops that may be fed to animals or humans. Due to the potential for herbicides with no established residue tolerances entering the food chain, it is essential to follow all rotational restrictions on labels when planting cover crops that may be grazed or harvested for forage. In situations where the cover crop is only used for conservation practices, greater flexibility is provided although the grower assumes the risk if the herbicide should interfere with the establishment of the cover crop. As cover crops are planted more widely, labels likely will change to take them into account. For example, the Harness label now states that only non-food or non-feed winter cover crops may be planted following harvest of food crops treated with Harness.

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Will herbicide interfere with establishing the cover crop?
The potential for herbicides to prevent successful establishment of cover crops is an important consideration. The threat posed by a herbicide is determined by the chemical's half-life and availability in the soil, sensitivity of the cover crop species, herbicide application rate and date, and environmental conditions throughout the growing season. Late herbicide applications and limited rainfall following application will increase the potential for crop injury. The relatively short time period between cover crop planting dates and the onset of cool fall temperatures increases the risk that herbicides pose to cover crops.

Anderson and Hartzler evaluated the response of five cover crop species to several persistent herbicides commonly used in Iowa corn and soybean production. All experiments were conducted in the greenhouse, thus the studies provide information on the relative tolerance of the five cover crop species to the herbicides rather than an assessment of actual risk under field conditions.

Study looked at tolerance of 5 cover crop species to herbicides
Results of the greenhouse trials are summarized in Table 1. The ratings are based on both greenhouse experiments and experiences with herbicides in the field. Radish was the most sensitive of the cover crops evaluated, with significant injury occurring with all herbicides except Dual II Magnum and Prowl. Cereal rye was the most tolerant of the cover crops. Hornet caused serious injury to plant death on the three broadleaf species, whereas Corvus affected the growth and vigor of all species.

Table 1. Relative tolerance of several cover crop species to herbicides commonly used in corn and soybean production. Injury potential ratings are based on greenhouse trial.

There are many benefits associated with inclusion of cover crops into the corn/soybean cropping systems that dominate the Iowa landscape.

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"Our relatively short growing season in Iowa limits the time period for growth of cover crops following planting and the onset of dormancy, thus increasing the threat posed by herbicide residues," says Hartzler. "Cereal rye has a relatively high tolerance to the herbicides commonly used in corn and soybean production, and under most situations its establishment should not be affected by herbicides used earlier in the growing season. Other cover crop species are more sensitive to herbicides, and the potential impacts of herbicides on their establishment should be considered."

Final thought: Always follow any rotational restrictions on the herbicide label when cover crops might be harvested for forage or grazed.

Bob Hartzler is a professor in the Department of Agronomy at Iowa State University. Hartzler can be reached at [email protected] or 515-294-1164. Meaghan Anderson is an Extension field agronomist serving east-central Iowa. Anderson can be reached at [email protected] or 319-337-2145.