Proactive action controls spread of weed

AgResearch science is leading the way in preventative action against an extremely costly weed.

Careful management is the key to protecting dairy farmer incomes from the effects of the giant buttercup weed, says AgResearch scientist Dr Graeme Bourdôt.

Dairy farmers can lose $1,040 per hectare per year in bottom line profit with just a 12% infestation of giant buttercup in their pastures. The loss in revenue results from the combined costs of the lost grazing area and avoidance by the cows of the pasture surrounding the buttercup plants.

Giant buttercup is established in Tasman/Golden Bay, West Coast, Horowhenua, Hawke’s Bay, Taranaki and South Auckland and has been in New Zealand for well over 100 years. Until recently, central and southern South Island regions were thought to have minimal infestation.

Potential spread
A CLIMEX model for giant buttercup was developed by Graeme Bourdôt and fellow AgResearch weed scientist Shona Lamoureax in collaboration with Dr Mike Watt from Scion and Dr Darren Kriticos from CSIRO, and was published in the American Weed Science journal in April 2013. Developed to understand the potential further spread, it revealed that all of New Zealand is climatically suitable for this weed and that irrigation in the drier eastern parts of the country increases the suitability of these areas.

The model revealed that dairy farms in Southland and Canterbury are at risk to invasion by giant buttercup and Graeme presented this message at the 2012 NZ Grasslands Association Conference in Gore.

Following this alert Environment Southland conducted a survey of roadsides and adjacent paddocks in early 2013 through the main dairying districts of Southland and found the weed already established along 90 km of the 1,000 km of roadside sampled. The buttercup was also found to be well established in dairy pastures.

Genetic resistance
Further complicating matters Dr Bourdôt and his team now have good evidence to show that giant buttercup has developed a genetically-based resistance to all classes of herbicide that have been used to control it in dairy pastures.

“Some dairy farmers have been telling us that herbicides they were applying were no longer effective on their giant buttercup,” he says.

“One of our Postgraduate students studied an infestation on a farm in Golden Bay that had been treated for a number of seasons with one of the acetolactate synthase inhibitor herbicides. She found five times more herbicide was needed to control this infestation than for giant buttercup on a roadside without previous exposure to the herbicide.”

This is the second time in 20 years that the giant buttercup has been found to have developed resistance to a previously effective herbicide. In the early 1990s the plant became resistant to the phenoxy group of herbicides (MCPA; MCPB; 2, 4-D). Since then, farmers have been using the ALS group to control the weed but these herbicides are expensive and becoming less effective.

The plant spreads by both seed and rhizome fragments, which further complicates matters. Even when they are effective, herbicides can only ever be one of the tools needed for successful management of giant buttercup.

“For instance it would be good practice not to bring cows, hay or other materials onto your farm that have come from a farm infected with giant buttercup,” says Dr Bourdôt.

“That is why farmers in all dairy areas in the country need to be careful about their own biosecurity measures to make sure they don’t introduce the giant buttercup inadvertently.

“For those farms with an infestation, management options include a good pasture renewal or cropping strategy, careful pasture management to avoid the winter pugging that promotes the establishment and spread of the weed and avoidance of further resistance building in the weed by using different types of herbicide with each application.”

Farmers can find the latest control advice on the giant buttercup page on Agpest.

Environment Southland has been proactive regarding the threat that giant buttercup poses to dairying in Southland and with the assistance of industry stakeholders including AgResearch, is developing an industry-led management and awareness programme for the weed.

Southland dairy farmers now have the opportunity to avoid the huge economic losses that are attributable to giant buttercup in Taranaki and Golden Bay by implementing a proactive plan to control the existing infestations.