An Environment Southland survey of 900km of rural roadsides earlier this year found the perennial plant along 90km of it, which was ''clearly a warning signal'', the regional council's biosecurity manager, Richard Bowman, said.
Of European origin, giant buttercup occurred in dairy pastures in the South Auckland, Hawkes Bay and Taranaki regions and in South Wairarapa, Horowhenua and Tasman.
AgResearch estimated it could reduce a typical Golden Bay dairy farm profit by 36% through reduction in utilisable pasture dry matter.
The cost of production loss alone had been estimated at up to $1030 per hectare a year, while overall, it was responsible for an industry loss of up to $150 million annually in milk solids revenue.
This month, Environment Southland hosted a meeting attended by Federated Farmers, DairyNZ, AgResearch, the Southland District Council and farmers from known affected areas in Southland and Golden Bay.
The regional council was not prepared to lead on the issue but was happy to facilitate and co-ordinate a proactive approach.
''It is a dairy industry issue and they need to take the lead on it,'' Mr Bowman said.
It was not known how much had got into pasture in Southland. It was unpalatable to cows, he said.
Giant buttercup would have been in Southland ''for a very long time''. What was surprising was how much more widespread it was than was thought and farmers were in ''quite a bind'' over how to deal with it.
The regional council was aware of it in four areas in Southland - at Pourakino, Kennington, Mandeville and a small patch near Lumsden. Those areas were isolated and he was ''not sure why they are where they are''.
He believed awareness among farmers in Southland was ''probably quite low at present'' because it had not started to impact on them.
Three action points to address the issue were agreed on - an education and awareness campaign for farmers, more trials along roadsides to determine how to manage the spread, and research into better control methods.
The initiatives would be led by an industry steering group and it was likely government funding to assist would be sought from the Ministry for Primary Industries' sustainable farming fund.
Giant buttercup was very difficult to get rid once it was established and it quickly became resistant to chemical spraying.
Farmers who suspected giant buttercup was in their pasture should contact Environment Southland for verification and best practice control advice.
