A Foundation for Arable Research project has found that up to half the reported cases of weeds showing resistance to the chemical glyphosate are not what they seem.
The three year project has financial support from the Sustainable Farming Fund, Dairy NZ, the Research and Innovation Board and several agricultural chemical companies.
Glyphosate resistance was identified in New Zealand for the first time last year, in annual ryegrass in five Marlborough vineyards. The research project aims to find ways to avoid resistance to the herbicide.
Project team leader Mike Parker says resistance is usually linked to overuse of the chemical.
However he says poor application techniques may in some cases be to blame for weeds' survival rather than resistance.
"Glyphosate is absorbed through the green material, so if you have a very dense canopy of other weeds you may get a particular weed growing underneath the canopy that isn't covered by the chemical, and so it survives."
Mr Parker says people can wrongly assume this is resistance to the chemical rather than misapplication.
He says the project is looking at putting out some best management practices for the application of glyphosate.
Mr Parker says where there is still green material after spraying, it should be resprayed.
If the plant remains green after a second application, the foundation should be contacted, and it will take samples for analysis.