A United States jury has found glyphosate-based weed killer Roundup was a "substantial factor" in causing a man's cancer, the second such court ruling against the herbicide in less than a year.

Key points: A jury has found Roundup caused 70-year-old Edwin Hardeman's non-Hodgkin's lymphoma

A jury has found Roundup caused 70-year-old Edwin Hardeman's non-Hodgkin's lymphoma Bayer, which owns the company that makes Roundup, says it is confident the evidence in phase two of the case will show it should not be liable

Bayer, which owns the company that makes Roundup, says it is confident the evidence in phase two of the case will show it should not be liable Nufarm, which sells glyphosate herbicides in Australia, has reported a half-year loss

The unanimous decision in the US District Court in California found Roundup caused 70-year-old Edwin Hardeman to develop non-Hodgkin's lymphoma after he used it for nearly 30 years to control weeds on his property.

However, the case now moves to a second stage, which will determine liability and damages.

Bayer, the owner of Monsanto, which makes Roundup, said in a statement that it was disappointed with the jury's initial decision.

"We continue to believe firmly that the science confirms glyphosate based herbicides do not cause cancer," the company said.

"We are confident the evidence in phase two will show that Monsanto's conduct has been appropriate and the company should not be liable for Mr Hardeman's cancer."

Mr Hardeman's lawyers said they would show evidence of Monsanto's "bad conduct" in the next stage of the trial.

Monsanto has strongly defended Roundup with a decades-long public relations campaign.

Last year, another jury found that Roundup had caused the cancer of a school janitor in California.

He was awarded $US289 million, but that was later reduced to $US78 million.

Bayer is appealing the case.

The current case is seen as a "bellwether" for hundreds of other pending lawsuits in the US.

In total, Bayer is facing more than 11,000 similar cases in the US.

Nufarm defends glyphosate as it announces loss

Australian farm chemical firm Nufarm is also maintaining the safety of glyphosate as it released its half-year results.

It said regulators in Canada and Brazil had confirmed regulatory approval of the herbicide after re-evaluating the product.

"Both authorities reaffirmed that glyphosate is safe to use and presents no risk to users when used in accordance with label instructions," Nufarm said in its financial results report.

Dry weather in eastern Australian and Europe hit Nufarm's bottom line over the first half of the 2019 financial year as farmers bought less fertiliser and herbicide like glyphosate.

It made a net loss of $13.6 million for the half-year to the end of January and did not pay investors a dividend.

Glyphosate sales fell 9 per cent from the same a year ago because of extremely dry conditions across much of Australia.

Nufarm shares plunged 24 per cent to $4.23 as the company said it was cutting production in Australia and New Zealand by around half and slashed its earnings forecast for 2019.