Warning on local weed imports

That’s the message from Agresearch scientist Graeme Bourdot following on-farm finds of giant buttercup in Golden Bay resistant to both herbicide groups registered for its control. The weed, already present in six of New Zealand’s 17 dairy regions, and climatically suited to grow nationwide, can cause substantial production losses.

Bourdot and colleagues calculate the impact on the 100ha Golden Bay farm’s bottom line to be $100,000.

“Losses are proportionate to the ground cover of the buttercup plus 25%. It’s more than just the area the buttercup occupies because the cows don’t graze up close to it because the smell of it repels them.”

Widespread resistance to phenoxy mode-of-action herbicides such as MCPA and MCPB was found in giant buttercup in the early 2000s, leaving ALS-inhibitor flumetsulem as the only herbicide option in such cases. Now, repeated use of that mode of action has seen the giant buttercup population on at least one farm develop resistance such that even a dose of five times the label rate only kills 90% of seedlings.

“We don’t know how widespread geographically the resistance to flumetsulam is, but where it’s been used repeatedly there could be resistance now. Once-per-season [use] is all that’s required [to select for resistance].”

The weed, in New Zealand for at least a century, is a big problem in Taranaki and Golden Bay, and is creeping down the South Island West Coast. It’s also present but less widespread in Waikato, Horowhenua and Wairarapa.

“One simple thing you can do is if you are buying stock, don’t buy them from a farm that’s infested. Or if you do have to buy in stock from an infested farm keep them on a run-off area for a couple of days so any seed has passed through their gut and you can monitor that area for buttercup.”

Hay is also a possible means of import. Well made silage should be safe, as the heat should destroy the weed’s seeds, but research is needed to be sure of this, he adds.

“The seed is not windblown so the only way it is going to be moved is by farmers and farm produce from infested regions.”

Industry-wide, Bourdot calculates the weed is already costing millions in lost revenue, and should it spread to all 17 dairying regions national dairy farm revenue loss caused could hit $328 million to $748 m per year.

Recent work by AgResearch technician Carolyn Lusk at Lincoln confirmed the flumetsulam resistance.

“The AgResearch work also gives a possible explanation for the persistence of the weed on dairy farms in Golden Bay and in other regions where farmers have relied on regular applications of this herbicide to control the buttercup,” says Boudot.