WIith an increasing focus on reducing chemical herbicides, largely because of crop resistance and a potential build-up of residues, new methods of weed control are appearing.
Found throughout New Zealand, Tutsan is unpalatable to livestock but can be harmful if ingested. Like St John’s Wort, it contains hypericin, which causes photosensitisation and extreme skin sensitivity (hyperaesthesia) in sheep, cattle and horses.
In the past tutsan has been held in check by tutsan rust, but in the central North Island the plant is showing resistance to rust and spreading rapidly across hill country.
“The biggest problem with controlling tutsan at the moment is that the most effective herbicides are toxic to clover, which is counter-productive in the type of country where this weed is an issue,” says George Kerse, Ravensdown’s business manager agrochemicals.
“In the absence of other solutions such as biological control, some farms might have to use chemical controls and sacrifice clover growth, although the fact that there is no damage to grass lessens the impact,” he says. It can take 6-12 months for clover to re-establish after spraying.
Products that contain triclopyr and picloram appear to be the best available option at present, but these still require repeat treatments.
Biological control options are still being explored by the Tutsan Action Group, founded in 2011 and supported by the Sustainable Farming Fund. Tutsan Action Group chair Graham Wheeler says progress on biological controls is promising, but still a way off.
“The most important thing is that farmers need to deal with the weed as soon as it appears,” he says.
“Tutsan isn’t only a problem on poor land and if outbreaks are not dealt with as soon as possible, the weed will spread.”
The study says it is important to control isolated plants and to re-plant bare sites with more desirable, competitive species. Improving fertility can help prevent seedling growth and this needs to be supported with the application of fertiliser, oversowing with desirable species and stock grazing.
“The reality is that there is no silver bullet available at the moment which can solve the tutsan problem,” Kerse says.
“While we will continue to hope for a breakthrough biological treatment, at the present chemical control supported by sound management practices are the only option.”