Profit-strangling weed is herbicide immune

TIM CRONSHAW
Last updated 10:25 18/05/2012

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A giant buttercup that has yet to reach vulnerable Canterbury dairy pastures is not only costing farmers millions of dollars in lost revenue but has become a bigger threat after developing resistance to modern herbicides.

The bitter-tasting weed is avoided by cattle and has extended as far south as Tasman's Golden Bay, with the potential to spread to the east coast of the South Island, particularly on farms with irrigation.

AgResearch scientists are warning that nearly all of New Zealand is climatically ideal for the yellow-flowered weed and it is an internal bio- security risk to the dairy industry.

Group project manager Graeme Bourdot said the Canterbury Plains was among climatically suitable areas vulnerable to the weed, but some farms would be less exposed because they did not suffer from pugging which allowed it to quickly seed and become established in gaps in the soil.

He said the buttercup was a major problem for dairy pastures.

"I would rate it way up there as a dairy pasture weed. It's certainly one of the most significant dairy weeds in New Zealand," Bourdot said.

The buttercup is found in South Auckland, Hawke's Bay, Taranaki, South Wairarapa, Horowhenua and Tasman.

In Golden Bay, AgResearch worked out a model showing a $1040 loss in profitability from a typical infestation of the buttercup covering 12 per cent of a 100-hectare dairy farm split into 1ha paddocks.

The net $2870/ha profitability was reduced to $1830 in the study funded by DairyNZ.

About 98 per cent of the paddocks contained the buttercup, with the weed covering the worst by 40 per cent.

Earlier research showed giant buttercup cost Kiwi dairy farmers $156 million through lost feeding in the 2001-02 milking season.

Further modelling showed that, if the weed was to spread to all dairying regions, the total loss would be $328m to $748m a year.

Scientists believe the weed reached New Zealand from seed lines or on material from Europe arriving with early immigrants.

They have been researching the buttercup only since the mid-1980s and have yet to get a fix on the rate of its spread, although there is evidence in Golden Bay that it is most likely being carried and dispersed by farm machinery and stock. New studies by AgResearch technician Carolyn Lusk at Lincoln show giant buttercup is one of only a few pasture weeds worldwide to have developed resistance to herbicides,

Bourdot said this was a result of farmers inadvertently selecting plants for resistance by using the same herbicide for many years.

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He said farmers were being recommended to mix herbicides, but there were only two modes of action and there was good evidence now that the weed was resistant to both.

"The dilemma is there isn't an alternative mode of action," he said.

Bourdot said scientists were looking at alternative herbicides in North America not available in New Zealand and this had provided some hope.

The other glimmer of light was there were reports of farmers controlling the buttercup with "bucket chemistry" by combining herbicides with pasture growth promoters.

AgResearch has plans to begin a new research programme testing alternatives next season, subject to securing funding.

The yellow flowering pest, peaking in bloom in November, has bypassed sheep paddocks.

- The Press

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