Monsanto Co. and nearly a dozen state and national farm groups are suing California over the state’s decision to list the popular ag chemical glyphosate as a carcinogen.

“The impact on farmers could be … very detrimental,” said Kirk Leeds, CEO of the Iowa Soybean Association.

The soybean group, the Agribusiness Association of Iowa, and the National Corn Growers Association are among the groups suing California over its decision to add glyphosate to its list of cancer-causing chemicals.

Glyphosate is a key ingredient in Monsanto’s top-selling weedkiller Roundup.

The California Office of Environmental Health Hazard Assessment made the decision in July.

The agency said Wednesday it followed proper procedures in listing the herbicide and “stands by its decision.”

California’s Proposition 65, a ballot initiative passed in 1986, requires the state to protect drinking water sources “from being contaminated with chemicals known to cause cancer, birth defects or other reproductive harm.”

It also requires businesses to warn California users about their chemicals’ dangers.

The decision would not only hurt Monsanto, the St. Louis company said, but also “crops grown by U.S. farmers who use the herbicide, and food products derived from those crops.”

“Products with even trace residues of glyphosate” sold in California could be required to add a label in 2018 that Monsanto says would be “false and misleading.”

Leeds, the soybean association CEO, said the classification is inaccurate and its listing could be “a devastating blow to Iowa soybean farmers and an industry valued at more than $5 billion.”