A Northern California judge has upheld a jury's verdict that found Monsanto's glyphosate-based weedkillers, including Roundup, caused a groundskeeper's cancer, but she slashed the amount of money to be paid to the man from US$289 million ($441.4m) to US$78m ($119m).

In denying Monsanto's request for a new trial, San Francisco Superior Court Judge Suzanne Bolanos cut the jury's punitive damage award from US$250m to US$39m. The jury awarded punitive damages after it found the St Louis-based agribusiness had purposely ignored warnings and evidence that its Roundup product causes cancer, including DeWayne Johnson's lymphoma.

But in a tentative ruling on October 11, Bolanos said she was considering wiping out the US$250m judgment after finding no compelling evidence presented at trial that Monsanto employees ignored evidence that the weedkiller caused cancer.

DeWayne Johnson reacts after hearing the verdict in his case against Monsanto at the Superior Court in San Francisco. Photo / AP
DeWayne Johnson reacts after hearing the verdict in his case against Monsanto at the Superior Court in San Francisco. Photo / AP

The judge reversed course yesterday and said the jurors appeared to agree with Johnson's expert witness, Dr Chadi Nahban, who concluded that Monsanto's popular Roundup weed killer caused the groundskeeper's cancer.

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Some jurors were so upset by the prospect of having their verdict thrown out that they wrote to Bolanos. Gary Kitahata told the judge "to respect and honour our verdict and the six weeks of our lives that we dedicated to this trial".

Robert Howard said the jury paid "studious attention" to the evidence and any decision to overturn its verdict would shake his confidence in the judicial system.

- AP