San Francisco (AFP) - A lawyer for a California groundskeeper dying of cancer took aim at Monsanto Monday as a jury began hearing the lawsuit accusing the chemical giant of ignoring health risks of its top-selling weed killer Roundup.

"For the past 40 years, Monsanto has known the primary ingredient in Roundup can produce tumors in lab animals," attorney Brent Wisner told a California state court.

A jury is hearing the case brought by Dewayne Johnson, a 46-year-old father of two.

Diagnosed in 2014 with non-Hodgkin's lymphoma, a cancer that affects white blood cells, Johnson used a Monsanto generic version of Roundup called "Ranger Pro" repeatedly in his job at a school in Benicia, California, after being promoted to groundskeeper in 2012.

In his opening statement, Wisner said Monsanto opted against warning consumers of the risks and that instead "they have fought science" by playing down the suspected link between the chemical herbicide and cancer.

"Monsanto has gone out of its way to bully scientists and fight researchers," he told the jury.

The case in California Superior Court is the first trial in which Roundup is said to have caused cancer, a claim repeatedly denied by the chemical company.

If Monsanto loses, the case could open the door to hundreds of additional lawsuits against the company recently acquired by German-based pharmaceutical and chemical group Bayer.

- 'Told you could drink it' -

Johnson had little warning about the risks of Roundup, his lawyer said.

"He was told you could drink it, it was completely non toxic," Wisner said with his client sitting in the San Francisco courtroom.

"You will hear testimony from him that he got drenched in it, repeatedly."

The lawyer said Johnson, who is between rounds of chemotherapy, "is actually on borrowed time, he is not supposed to be alive today."

A key to Johnson's case will be convincing jurors that Monsanto's pesticide -- whose main ingredient is glyphosate -- is responsible for the illness. Wisner contended glyphosate combined with an ingredient intended to help it spread over leaves in a cancer-causing "synergy."

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Whether glyphosate causes cancer has been the source of long debate among government regulators, health experts and lawyers.

Monsanto has always denied any link with the disease and says studies have concluded the product is safe.

"Mr Johnson's cancer is a terrible disease. We all do and we all should have great sympathy for what he is going through," Monsanto defense attorney George Lombardi said during opening remarks in court.

"The scientific evidence is overwhelming that glyphosate-based products do not cause cancer, and did not cause Mr Johnson's cancer."

Roundup has been approved the US Environmental Protection Agency under various presidential administrations and through generations of scientists there, according to Lombardi.

Both sides at what is expected to be a month-long trial promised experts would testify to back their positions, and criticized what they saw as flaws in studies relied on by the other side.

"Mr Johnson's cancer began years before he took on his job with the school district," Lombardi contended.

- Cases 'in the pipeline' -

Robert F. Kennedy Jr -- an environmental lawyer, son of the late US senator, and a member of Johnson's legal team -- sat in the front row of the courtroom gallery.

"I don't think it's a surprise that Monsanto tried to stop the public from knowing about it, and tried to manipulate the regulatory process," Kennedy told reporters outside court.

Kennedy said his law firm has 700 clients "in the pipeline" with Roundup cancer cases.

When asked how much money he thought Johnson should get from Monsanto, Kennedy questioned whether $50 million or $100 million would even be enough.

- Agent Orange -

Founded in 1901 in St Louis, Missouri, Monsanto began producing agrochemicals in the 1940s. It was acquired by Bayer for more than $62 billion in June.

Monsanto was one of the companies which produced the defoliant "Agent Orange," which has been linked to cancer and other diseases, for use by US forces in Vietnam. It denies responsibility for how the military used it.

Monsanto's flagship herbicide Roundup was launched in 1976. The company soon thereafter began genetically modifying plants, making some resistant to Roundup.

In 2015, the International Agency for Research on Cancer -- a World Health Organization body -- classified glyphosate as "probably carcinogenic," and as a result the state of California listed it as carcinogenic.

But European food safety and chemicals agencies have so far not followed suit, while a US Department of Health study suggested its toxicity is limited.