Monsanto on Tuesday said it was asking a US appeals court to toss out a damning verdict in a landmark Roundup weed-killer cancer trial and grant it another hearing.

"By its appeal from the judgment and amended judgment, Monsanto also seeks appellate review of the trial court's order denying Monsanto's motion for new trial," an attorney for the agrochemical giant said in a filed notice-of-appeal.

The notice is the first step of the appeal process.

A cancer-stricken groundskeeper last month accepted a reduced damages award in the trial, which focused on weed-killer Roundup and a professional version of the herbicide called Ranger Pro.

Judge Suzanne Bolanos, who presided over the case in California state court, denied Monsanto's request for a new trial but cut the $289 million damages award to $78 million to comply with a law regarding how punitive damages awards must be calculated.

Bolanos denied a request by Monsanto to toss out the jury's overall verdict against the company.

Jurors in August unanimously found that Monsanto acted with "malice" and that its weed killers Roundup and Ranger Pro contributed "substantially" to Dewayne "Lee" Johnson's terminal illness.

The jury ordered Monsanto to pay $250 million in punitive damages along with compensatory damages and other costs, bringing the total figure to nearly $290 million.

Johnson, a California groundskeeper diagnosed in 2014 with non-Hodgkin's lymphoma (NHL) -- a cancer that affects white blood cells -- says he repeatedly used Ranger Pro while working at a school in Benicia, California.

Monsanto parent company Bayer reiterated in a statement Wednesday that "none of the science presented at trial supports the conclusion that glyphosate or the Roundup formulation was a substantial cause of Mr. Johnson's NHL".

The German group also said it would next month publish summaries of its own studies into glyphosate, the active ingredient in Roundup and Ranger Pro, on an online "transparency platform".

- More to come? -

Johnson's lawsuit built on 2015 findings by the International Agency for Research on Cancer, part of the UN World Health Organization, which classified glyphosate as a probable carcinogen, causing the state of California to follow suit.

Monsanto has defended the weed killer, arguing that it has a history of safe use that dates back more than 40 years.

The lawsuit is the first to charge the product with causing cancer, but observers say a Monsanto defeat likely opens the door to thousands of other claims against the company, which was recently acquired by Bayer.

Roundup is Monsanto's leading product.

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 launched Roundup in 1976.