Four more cases of glyphosate-resistant ryegrass have been found in vineyards in Marlborough, prompting concerns that the problem is more widespread than first thought.

These four cases were being investigated, AgResearch's Trevor James said.

Since the initial discovery of the resistant ryegrass in December more reports of weeds surviving glyphosate treatment had been coming in from all around New Zealand.

"All of these cases need to be investigated, although it is likely that many will be the result of application misses or errors, rather than resistance," he said.

When glyphosate was applied in the wrong conditions, or when spray penetration was insufficient to reach below canopy plants, this was counted as ‘glyphosate failure'.

A key part of the research project was the development of clear, sector-specific recommendations for the use of glyphosate.

He hoped that these best management practices would reduce the number of glyphosate failures and also the number of cases of resistance, which is generally linked to overuse of the chemical.

FAR project leader Mike Parker said there had been anecdotal reports of glyphosate resistance being found in Canterbury.

"Currently we can't confirm any resistance in Canterbury, but that's not to say it's not there," he said.

They were also testing samples of roadside weeds from the West Coast. This was on the advice from Australian scientists who had urged them to test roadside plants after they found resistance in weeds due to regional authorities repeatedly using glyphosate.

From there it had spread from the fencelines and into crops.

"We've got the regional councils on board," he said.

Testing for resistance took about three months, as individual plants had to be transported to a quarantine laboratory, split into tillers, grown out, and then treated with varying rates of glyphosate.

"It's not a simple process."

His biggest concern was that the resistance would spread to a plant species that was easily dispersed by the wind, allowing it to spread quickly around the country.

Farmers who suspected glyphosate resistance should re-spray the affected plant or plants, recording the rates of glyphosate used, and then, if the plant survived, contact the project team for further information on how to submit plants for official testing.