SAN FRANCISCO -- A U.S. jury on Wednesday awarded more than US$80 million in damages to a California man who blamed Roundup weed killer for his cancer, in a case that his attorneys say could help determine the fate of hundreds of similar lawsuits.

Edwin Hardeman proved that Roundup's design was defective, it lacked sufficient cancer warnings and its manufacturer, agribusiness giant Monsanto, was negligent, the six-person jury in San Francisco found.

It awarded Hardeman more than US$5 million in compensation and an additional US$75 million in punitive damages. Hardeman, 70, put his arm around his wife, Mary, as the verdict was read and hugged his attorneys.

Monsanto says studies have established that glyphosate, the active ingredient in its widely used weed killer, is safe. The company said it will appeal.

"We are disappointed with the jury's decision, but this verdict does not change the weight of over four decades of extensive science and the conclusions of regulators worldwide that support the safety of our glyphosate-based herbicides and that they are not carcinogenic," according to a statement from Bayer, which acquired Monsanto last year.

Hardeman said he used Roundup products to treat poison oak, overgrowth and weeds on his San Francisco Bay Area property for years. The same jury previously found that Roundup was a substantial factor in Hardeman's non-Hodgkin's lymphoma.

"Today, the jury sent a message loud and clear that companies should no longer put products on the market for anyone to buy without being truthful, without testing their product and without warning if it causes cancer," said Jennifer Moore, one of Hardeman's attorneys.

Hardeman and his wife thanked their attorneys and jurors but declined additional comment.

A different jury in August awarded another man US$289 million, but a judge later slashed it to US$78 million. Monsanto has appealed.

Hardeman's trial may be more significant than that case. U.S. Judge Vince Chhabria is overseeing hundreds of Roundup lawsuits and has deemed Hardeman's case and two others "bellwether trials."

The outcome of such cases can help attorneys decide whether to keep fighting similar lawsuits or settle them. Legal experts said verdicts in favour of Hardeman and the other test plaintiffs would give their attorneys a strong bargaining position in any settlement talks for the remaining cases before Chhabria.

Bayer says all government regulators that have looked at the issue have rejected a link between cancer and glyphosate.

Monsanto developed glyphosate in the 1970s, and the weed killer is now sold in more than 160 countries and widely used in the U.S.

The herbicide came under increasing scrutiny after the France-based International Agency for Research on Cancer, which is part of the World Health Organization, classified it as a "probable human carcinogen" in 2015.

Lawsuits against Monsanto followed, and thousands are now pending nationwide.

Monsanto has attacked the international research agency's opinion as an outlier. The U.S. Environmental Protection Agency says glyphosate is safe for people when used in accordance with label directions.