AgResearch principal scientist Graeme Bourdot said the $1.6b estimate didn’t include control costs, the loss of production from all 187 problematic pasture weeds, or the effect from the spread of unwanted plants.
“It’s a big number – and the real one is even bigger,” Bourdot said, although he was reluctant to be drawn on what it could be.
Researchers from AgResearch and Scion, with economists from Lincoln University’s Agribusiness and Economics Research Unit, used published papers to calculate the economic cost.
Weeds in wheat, barley, maize grain, and herbage seed arable crops were calculated to cost $18.2 million in lost production, and in forestry, $333.6m.
Lost pasture production from gorse, California thistle, sweet brier, hawkweed, broom, giant buttercup, yellow bristle grass, Chilean needle grass, nassella tussock and blackberry was calculated at $1.3b.
The weeds having the biggest effect on pastoral production were California thistle at $704m, yellow bristle grass at $258m, and giant buttercup, $210m.
Bourdot said the study didn’t calculate control costs – he was aware of a large North Island East Coast farm that spent $100,000 a year spraying thistles.
Researchers calculated that if giant buttercup occupied its entire potential range in 20 years, it would cost the dairy industry $592m.
Bourdot said reliance on herbicides was not a long-term solution, with thistles and buttercup showing signs of resistance.
“We are losing efficacy with chemical herbicides against many pasture weeds.”
Orion AgriScience has detected an incidence of early-stage resistance to the herbicide nicosulfuron to summer grass, a weed affecting maize paddocks.
Massey University’s senior lecturer in weed science, Kerry Harrington, who was also the convenor of the New Zealand Herbicide Resistance Task Force, said tests of seed from a Bay of Plenty farmer showed a two-fold level of resistance.
“This means that twice the usual amount of chemical is required for control, which probably means the strain is at the early stage of resistance,” he said.
Research manager for the Foundation for Arable Research, Mike Parker, said growers should alternate between different modes of chemical action groups when the same crop was grown in the same paddock in consecutive seasons, or rotate crops that used a different chemistry.
Herbicide resistance was a significant issue with California thistles in North America, and Bourdot said it was inevitable it would happen here.
Of the 63 thistle species in NZ, nine were problematic but the cassida beetle, which strips thistles of foliage, was showing promise. Similarly, natural agents were proving successful targeting gorse and broom in North Canterbury.
There were few options for giant buttercup, with fears a biological control would also target native buttercup species.
“We’ve put that in the too-hard basket, but the thistle is one where we are making real progress with,” Bourdot said.
He said the research was a timely reminder about the need to continue to invest in weed research, and he urged landowners to advocate for funding and to help spread biological control agents.
Community support had worked with dung beetle and wilding pines, with the Government pledging $16m over four years to the National Wilding Conifer Control Programme.
Massey’s Kerry Harrington said weed control wasn’t taken seriously by many, or was only focused where it was most visual such as footpaths, roads or streams.
This report was a timely reminder about the role of research to find new solutions, he said.
