AgResearch scientists have found one of the dairy industry's worst pasture weeds has become resistant to the herbicides used to control it.
Giant buttercup is costing dairy farming an estimated $150 million a year in lost income.
The Tasman and Taranaki regions are the worst infested, but the weed is also invading dairy farms in South Auckland, Hawke's Bay, Southern Wairarapa and Horowhenua and has the potential to spread through all dairying regions.
AgResearch group project manager Graeme Bourdot says earlier research in Golden Bay revealed giant buttercup was developing resistance to the MCPA herbicide that was in use at the time.
He says new research has confirmed that another class of herbicide introduced as an alternative, ALS Inhibitor, is also becoming ineffective.
Dr Bourdot says these are the only two classes of herbicide on the market that are registered for use against giant buttercup in dairy pastures, so that leaves New Zealand dairy farmers without a herbicide that is generally effective.
He says it could take five years or more to develop alternative chemical controls for buttercup.
In the meantime, the only other practical option open to farmers is to try to limit the spread of the weed by good pasture management.
Weed resistance is included in arable sector research projects to receive a boost from the government's Sustainable Farming Fund.
The Foundation of Arable Research, which represents growers of grass seed, cereal, herbage, and brassica crops, has pocketed more than a million-dollars for six projects.
The chief executive, Nick Pyke, says one of those is a programme which aims to stop weeds from developing resistance to commonly used herbicides such as Round Up,
Mr Pyke says a third of the new funding will go towards a project which aims to reduce the environmental footprint of arable crops.
Other projects to receive funding include the management of grass grub and building better biodiversity on arable farms.