A US jury has ordered Monsanto to pay $US2 billion ($2.8b) to a couple who contracted cancer after using the commonly used weedkiller, Roundup.

It is the third such case Monsanto has lost, and the biggest payout ordered so far.

The latest plaintiffs, Alberta and Alva Pilliod, used Roundup for decades and were both later diagnosed with non-Hodgkin's lymphoma.

“We wish that Monsanto had warned us ahead of time about the dangers of using Monsanto [weedkiller],” Ms Pilliod told reporters outside court.

“And that there was something on the front of their label that said; ‘Danger may cause cancer’.”

Lawyer Brent Wiessner said there would be more cases against the company.

“This is not the end, this is the beginning, there are tens of thousands of people out there, probably 30,000, or 40,000 people out there,” he said.

“And I've told Bayer and Monsanto we are coming after them.”

German firm Bayer bought Monsanto for $62.5 billion last year, making it responsible for the pay-outs against them.

A Bayer spokesperson called the jury's decision “excessive and unjustifiable”.

The chemical has been controversial in Australia too, but National Farmers Federation president Fiona Simson says she is confident in Australia’s rigorous approval process.

“In Australia, the registration process of all pesticides involves years of data collection and comprehensive assessment before approvals are granted and a product can be sold on the Australian market,” she said.

“Glyphosate is vital to modern day farming systems. Without it we couldn't farm as environmentally sustainably as we do now.”