In 2017, the state of California deemed glyphosate - the key ingredient in Monsanto's best selling herbicide, RoundUp, and many other pesticides used around the world - worthy of being added to its state list of chemicals that cause cancer. The decision would require RoundUp and other glyphosate products to bear a label warning potential buyers that its ingredients can cause cancer.

The vote to add glyphosate to the list, also known as Proposition 65, was influenced by a 2015 report from the World Health Organization's International Agency for Research on Cancer (IARC), which deemed glyphosate "a probable human carcinogen." But that decision continues to be challenged.

Inflamed by California's decision, a dozen national and midwestern agricultural groups, including Monsanto, took the state to court over the label, insisting that glyphosate does not cause cancer and that the label would be "false" and "misleading."

In contrast with the IARC's report, the EPA announced that the weed killer is not likely to cause cancer last month.

A senior scientist for the Center for Biological Diversity responded by telling the Los Angeles Times  tha t,  "the  only way the EPA could conclude that glyphosate poses no significant risks to human health was to only analyze industry studies and ignore its own guidelines when estimating cancer risk."

Controversy over the chemical remains intertwined with opposition to genetically

modified crops. Monsanto, which is merging with agrochemical giant, Bayer, has patented several different versions of major commodity crops, all of which are resistant to its patented pesticide, RoundUp.

The EPA's draft risk assessment on glyphosate may become a foundational document

in the pending extension of the product's registration for U.S. use in 2019.

The agency is set to open a 60-day public comment period on the risk assessment, beginning this month.