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Herbicide Residue Left After Drought Could Harm Winter Crops

The commonly used herbicide atrazine kills weeds in corn and sorghum fields without harming the crops.

winter wheat

Photo: Cindy Cornett Seigel (Flickr)

A farmer harvests a field of winter wheat in Indiana.

Purdue extension scientists say farmers who plan to grow late-season crops like winter wheat should check their fields for certain herbicide residues. Officials say the drought might have slowed the breakdown of some widely-used agricultural chemicals that can harm crops.

It is the nature of an herbicide to play well with certain plants and not with others. The commonly used herbicide atrazine kills weeds in corn and sorghum fields without harming the crops. But it does not do so well by other plants like soybeans or winter wheat.

Farmers who apply atrazine to a corn crop in the spring can normally rely on soil microbes to break the chemical down before they plant for winter. Purdue Weed Science Program Specialist Travis Legleiter says that’s if those microorganisms get the heat and water they need.

“This year we had the temperatures for those soil microbes but we didn’t have the moisture,” he says. “And they really depend on that moisture to be active, and with dry soils they’re not as active, and so those herbicides are going to persist longer than what we’re really used to them persisting in the soil.”

Legleiter says the extension discourages farmers from growing winter wheat on top of atrazine residues because it’s an “off-label” use of that herbicide.

Leo Reed is a licensing and certification manager with the State Chemist’s Office, which investigates herbicide misuse in Indiana. He says many of the more than 10,000 agricultural chemicals licensed in the state have official use labels that run for pages. That makes it hard to determine whether a farmer who planted winter wheat in fields with left over atrazine would be breaking the law.

“Off-label to us as regulators means that you’ve violated that product label,” Reed says. “And if you violate the product label, that’s also a violation of state and federal law. So you have off-label use, which is a violation. Then you have not recommended issues. That’s not a violation of the law.”

Legleiter says farms in parts of the state that have caught up on their rainfall probably won’t have a residue problem. And those growers who do find atrazine residue may also find their soil is still too dry to grow winter wheat.

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