A U.S. federal jury in San Francisco has awarded more than US$80 million to a man after determining last week that his exposure to Monsanto's widely-used weed killer Roundup was a significant factor in the development of his cancer.

The verdict announced Wednesday concludes the second and final phase of the trial in Edwin Hardeman's case against the company.

Jurors ruled again in favor of Hardeman, saying that Monsanto, which was purchased last year by Bayer AG for US$63 billion, caused the California man harm by failing to adequately warn of the potential dangers of using the glyphosate-based weed killer.

Hardeman was awarded US$75 million in punitive damages and about US$5.8 million in compensatory damages.

"As demonstrated throughout trial, since Roundup’s inception over 40 years ago, Monsanto refuses to act responsibly," Hardeman's attorneys Aimee Wagstaff and Jennifer Moore was quoted as saying in a joint statement. "Today, the jury resoundingly held Monsanto accountable for its 40 years of corporate malfeasance and sent a message to Monsanto that it needs to change the way it does business."

The development comes as a major blow for Bayer, whose shares tumbled in August last year when another California man was awarded US$289 million after a state court jury found Roundup caused his cancer. That award was later reduced to US$78 million and is on appeal.

Bayer said Wednesday it would also appeal the verdict in Hardeman's case. The German pharmaceutical company has continued to argue that the herbicide is safe.

"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," Bayer was quoted as saying in a statement.

"We have great sympathy for Mr. Hardeman and his family. Bayer stands behind these products and will vigorously defend them."