Judge Winifred Smith reduced the total award for Alva and Alberta Pilliod, who say the glyphosate contained in Roundup gave them non-Hodgkin lymphoma, from $US2 billion ($A2.9 billion) to $US86.7 million ($A124 million).

In a statement, Bayer, the German pharmaceuticals giant which owns Monsanto, said the ruling was a "step in the right direction" but added that the company continued "to believe that the verdict and damage awards are not supported by the evidence at trial."

Bayer also said it would "appeal on multiple grounds" and that there was no proof that glyphosate causes cancer.

"Significantly, the largest and most recent epidemiology studies ... each found that there was no association between glyphosate-based herbicides and NHL (non-Hodgkin lymphoma)," the statement said.

The judge rejected Bayer's request to have the fine cancelled completely. This means that even with the lower fine, the verdict still finds Bayer responsible for the Pilliods' cancer.

The Pilliods can now decide whether they will accept the reduced damages payment, or if they want a new trial. Their lawyer Brent Wisner called the decision a "great victory".

Bayer faces more than 13,400 lawsuits in the United States over alleged cancer risks from Roundup, after purchasing Monsanto in a deal that was finalised last year.

It has already lost three US trials including the Pilliods', which concluded in May. Smith had already said that the amount of compensation would be reduced in their case as it was unconstitutional.

Australian Associated Press