California to list glyphosate as cancer-causing; Monsanto vows fight. http://www.reuters.com/article/us-usa-glyphosate-california-idUSKBN19H2K1 Listing glyphosate as a known carcinogen under California's Proposition 65 would require companies selling the chemical in the state to add warning labels to packaging. Warnings would also be required if glyphosate is being sprayed at levels deemed unsafe by regulators.