Consider spring herbicide program prior to planting fall cover crops

Producers planning to use cover crops this fall need to be aware of what herbicides they applied this spring and what the restrictions are for seeding following application. 

According to Bob Hartzler, Iowa State University Extension Weed Specialist, the use of preemergence herbicides has increased in recent years due to the spread of herbicide resistant weeds. “Many of these products are persistent in the environment, and phytotoxic concentrations may remain in the soil at the time of cover crop establishment in the fall,” he said. “While herbicide labels provide information on restrictions regarding rotational crops, these recommendations generally are not written with cover crops in mind.”

Hartzler says restrictions on the label that prohibit planting cover crops may be due to:

1. the risk of herbicide residues causing establishment failures;
2. the risk of herbicide residues accumulating in the cover crop intended for livestock feed for which residue tolerances have not been established;
3. a lack of data determining the safety of the herbicide on the cover crop; and
4. a combination of the above factors.

Hartzler adds that our relatively short growing season limits the time-period for cover crop growth following planting and the onset of dormancy. “This increases the threat posed by herbicide residues. Cereal rye has a relatively high tolerance to herbicides commonly used in corn and soybean, and under most situations its establishment should not be affected by herbicides used earlier in the growing season,” he said. “Other cover crop species are more sensitive to herbicides, and the potential impacts of herbicides on their establishment should be considered.”

Josh Dee, Agronomist for IAS Cooperative in Waukon, states that some of the more common herbicides used this spring were Atrazine, Bicep (Metachlor and Atrazine), Harness Xtra, Triple Flex, Resicore, Acuron and HalexGT. Dee cautions that there are many factors that can impact the amount of carryover including application rate, when it was applied, and the amount of rainfall since application. “Herbicide carryover can also be impacted by soil texture, pH and organic matter content,” said Dee.

Missouri Study
According to a University of Missouri study on the effects of herbicide carryover on cover crops, completed from 2013 to 2015 in Columbia, the general order of sensitivity to cover crops to herbicide carryover, from greatest to least, is: tillage radish>Austrian winter pea>crimson clover=annual ryegrass>winter wheat=oats>hairy vetch=cereal rye.

The study concluded that the most injurious herbicide treatments following soybeans were: fomesafen (Flexstar/Prefix), imazethapyr (Pursuit), acetochlor (Warrant), and sulfentrazone (Authority products). 

They found the most injurious following corn were: topramezone (Impact), mesotrione (Callisto, Halex Gt, etc.), clopyralid (Stinger, Surestart), isoxaflutole (Balance Flexx), and nicosulfuron (Accent Q, etc.).

LuAnn Rolling, District Conservationist for USDA’s Natural Resources Conservation Service in Allamakee County, says that if there is any question about carryover, a bioassay could be done using seeds to be grown.

The University of Nebraska has a publication that explains a bioassay procedure. The publication is available online at: http://extensionpublications.unl.edu/assets/pdf/g1891.pdf. The Allamakee Soil and Water Conservation District office can also assist you with proper procedure. 

Rolling adds that if there is cost-share lined up for cover crops in a field anticipated to have carryover, those acres must be cancelled or moved as soon as possible.

Livestock Feed Risk
When evaluating the risk of herbicides, Hartler says additional caution should be taken to the potential use of the cover crop. “If there is any possibility that the cover crop will be grazed or harvested for forage, all restrictions regarding rotational crops must be followed,” he said.

“This is necessary to prevent herbicide residues being fed to animals that are not cleared for consumption (residue tolerance).”

Hartzler added that if a cover crop is only used for conservation purposes, the grower can plant a cover crop that is prohibited on the label. However, the grower accepts all responsibility if the herbicide interferes with establishment of the cover crop in this situation.

A bulletin from the University of Wisconsin, “Herbicide rotation restrictions in forage and cover cropping systems”, WCWS 201, describes the rotation restrictions on labels of most commonly used herbicides. It is available at http://mccc.msu.edu/wp-content/uploads/2016/10/WI_2015_Herbicide-Rotatio...

Contact the Allamakee SWCD/NRCS field office for more information on herbicide carryover and impacts on cover crops.

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