The herbicide glyphosate, the active ingredient in Monsanto’s Roundup, has been added to California’s list of chemicals known to cause cancer. The listing, which goes into effect on July 7, will require manufacturers to add package warning labels and similar warnings to be places around certain sites where large quantities of the weed killer have been applied. The World Health Organization had previous labeled glyphosate as a “probable” human carcinogen. The chemical is the most widely used pesticide in California – and in the entire world. Monsanto has said it will fight California’s designation in court.