Rusty the collie heading dog has come all the way from Southland to help find the invasive weed velvetleaf on farms in the Horizons Regional Council area.

If there's a breeze, he can sniff out a seedling with just two leaves from 85m away.

He's only in the Manawatu area for three days, council pest plant co-ordinator Craig Davey said. Yesterday, the dog was on a Kauangaroa dairy farm and had found several plants by 10am.

Rusty was initially a search and rescue dog, and he was good. When his owner, Invercargill electrician John Taylor, heard about the arrival of the pest plant velvetleaf he trained Rusty to search it out.

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So far, Rusty is the only dog in the country that can do it. His expertise has also been used in Waikato and will be available elsewhere. Other dogs are being trained.

Rusty concentrates when he's working and Taylor can tell when he's under too much pressure, because he starts nibbling grass.

Watching Rusty work was Landcare Research researcher Fiona Thomson, who said people underestimate the usefulness of dogs. Davey said dogs would be cheaper than drones or heat sensing technology for searching out plants.

Thomson is looking for funding to compare the dogs' ability to humans', and she's training her own dog to search out Chilean needle grass, a pest in Hawke's Bay.

"You make it a game. Dogs naturally want to hunt out stuff, and they want to please you," she said.

The velvetleaf weed arrived in New Zealand last year, as a contaminant in Kyros brand fodder beet seed. It was quickly found, and measures were taken to eradicate it. But some has escaped.

The Horizons Region has the most infestations in New Zealand, Davey said. If the weed can't be eradicated then paddocks with it will have to be closed.

Velvetleaf is "a growing machine", each plant making thousands of seeds that last 50 years in the ground. It's unpalatable to stock and quickly takes over pasture.

Machinery used on infested farms has to be hosed down before moving elsewhere, and any plants and seeds found have to be sealed tightly and put into a contained area at Bonny Glen Landfill.

Farmers have mostly been brilliant at looking out for the weed, Davey said. Those who have had it will have to keep up the surveillance.