The Field Horsetail Challenge

A hugely invasive and sneaky weed, called field horsetail, is causing havoc in Manawatu but a farmer group is working to find natural predators to control the weed.

Farmers need to have their eyes open for this invasive weed, which is virtually impossible to kill with standard chemicals.  Field horsetail is often unknowingly transported in gravel or metal, and is not very noticeable when it first arrives.

Recently 85 people attended a Rangitikei Horsetail Group field day in Manawatu, which demonstrates not only awareness but also increasing numbers of people worried about the spread of field horsetail.

Farmers have formed a group to find ways to minimise the impact of the weed, gaining Sustainable Farming Fund backing for research to find biological controls.

Craig Davey, an environmental co-ordinator with Horizons Regional Council, describes the plant as a prehistoric monster, which has been around for a long, long time.  His advice to farmers is:

• Know what field horsetail looks like
• Look on your farm and around your neighbourhood for it
• Be aware of how it could have arrived on site
• Ask your contractors and gravel suppliers about it

Not only has the weed become a problem throughout hundreds of hectares of Manawatu, it has also spread into crops in Gisborne and Hawke’s Bay and is in river systems in Marlborough.

The weed disperses through root fragments and in the 2004 floods spread much more widely.  There is the potential for cropping and pastoral districts to be inundated as a result of fragments moved on machinery.  And it’s a significant threat to dairy pastures.

Craig says, “We are confident about saying when river gravels are harvested the roots of the plant, which are brittle and stick-like, are picked up by the machinery and then spread with the gravel.”

So when farmers use river-run rocks or base course metal for tracks, then field horsetail can easily spread alongside those tracks.

The weed problem has grown exponentially as a result.  “It is a plant with the potential to be quite damaging to a wide sphere of agriculture, including grassed areas and cropping land including maize.”

It is easily spread during cultivation, resulting in a paddock full of field horsetail.  And it’s poisonous to horses and stock. If hay bales contain only 15-20% of horsetail, when it rots it will promote fungus and ruin the rest of the hay.

Chemical and management trials have been undertaken with AgResearch, and it appears there is no simple chemical fix.

The Council’s message is they want people to be aware of it and know what to look out for.  Farmers have to be more vigilant about what comes onto their farms.  The spread can be slowed or stopped by cleaning machinery, and only receiving clean product.

The plant mines silica from the soil and is sometimes used in homeopathic remedies.  It looks green and lush but feels quite rough and abrasive.

A German sauna manufacturer has asked for supplies of up to 50kg/month from New Zealand, because of these abrasive properties.

Craig recommends if people buy builder’s mix they store it on hard plastic, and don’t put any leftover mix on the garden.  “It’s turned up twice at my place in the builder’s mix.”

In 2007 Horizons Regional Council added it to their pest plant strategy, under the “investigate” category.  “We can’t make people get rid of it because we can’t get rid of it,” he says.

Investigation is under way to identify the pathways by which the weed is spread and what can be done to minimise it.

AgResearch is looking at chemical control and the Landcare Trust is also involved in looking at the best methods to minimise the impact of horsetail on farms.  But nothing in isolation is effective, Craig says. “It’s very challenging that we aren’t able to remove it.”

“If we can’t prevent it being spread, and can’t control it, then we have to be serious about the best way to minimise its impact by looking at biological controls through insects or plant diseases.

The Rangitikei Horsetail Group was set up and obtained $316,000 from the Sustainable Farming Fund and $160,000 from farmers and local organisations to take on this issue. “That is sufficient for us to get European scientists to supply us with a shortlist of the best bugs and diseases.”

While it is a weed in both North America and Europe, it is more in ecological equilibrium compared to here.

Landcare Research is involved in this process to find a biocontrol agent.

Alistair Robertson’s dairy farm bounds the Rangitikei River for 3.5km where field horsetail is rampant on the river edges in amongst the bush, willows and toe-toe.  “When we first developed the property in 2007 we carted hundreds of tonnes of metal off the river to form our raceways.  By 2011 we started to see little patches popping up.  Then we started to notice it about the farm and I started hunting out information,” he says.

At a presentation by Horizons and the Landcare Trust, Alistair put his hand up to join a group to apply for SFF funding.  The three-year project involving Landcare Research is based at Lincoln University, and will cost around $480,000.

Other funding has come from farmers such as Alistair, as well as the NZ Biocontrol Collective donating a total of $60,000 over the three years.  The Rangitikei District Council and Horizons have also been very helpful.

Two sawflies already been imported from the Northern Hemisphere and are being trialed in a closed facility at Lincoln.  “We are also looking for a little flea beetle and a stem weevil which is proving elusive. We are hoping to look again in the Northern Hemisphere spring for these agents.”

The aim is to have four or five different biocontrol agents to trail by the end of year two.

Meanwhile the weed is creeping slowly into his dairy pasture but hasn’t yet made a very big impact on production.  “It is still reasonably confined in the river and stopbank corridor and along the drains where it has been fenced.”

Alistair estimates so far he has spent a few thousand dollars spraying it to contain it. But farmers with crops such as vegetables or onions are finding it much more difficult because herbicides can’t be used in amongst their crops.

“You can’t spray herbicides amongst forage crops or cereals.  This is where the biggest threat is now,” he says.

It can also push up through pavements and come through tarseal on sidewalks.  “It has a lot of grunt to push through that surface.”

“There are a few challenges ahead. We are only just getting started on this journey.”

Alistair is hoping for a biocontrol as effective as that on ragwort, where the cinnabar moth, ragwort flea beetle and ragwort seedfly have been effective control agents.  Alistair says the scientists are confident that in five or 10 years they hope to have suitable bio-control agents released to help get field horsetail under control.