Wet mowing kills weed

Californian thistle is NZ's most destructure pastoral weed and with its deep root system can be very difficult to control. The knowledge that better control is achieved if mown in the rain has been about farmers circles for years, and those that have irrigation often use this technique successfully.

Science has the job of quantifying why the kill rate is better when mowing in the rain and if a biological control method would be useful. This is a difficult weed to control and over the years many management techniques and sprays have been used with varying success. The use of farmer knowledge in solving problems is not utilised enough by science, and these revelations shows how practical ideas can provide successful solutions to major problems.

Research has provided evidence to show that mowing californian thistle in the rain really does help get rid of the weed reports The Taranaki Daily. It will come as no surprise to many farmers, but there is now evidence that mowing pasture in the rain helps to reduce the abundance of Cirsium arvense. It is the most destructive pastoral weed in NZ.

This work is the latest product of 20 years of collaborative research on weed control methods by a team of scientists from the Crown Research Institutes - AgResearch and Landcare Research, the Bio- Protection Research Centre, industry organisations and community groups.Project leader Graeme Bourdot, senior scientist at AgResearch Lincoln, said the finding emerged from a national survey funded by the former Meat and Wool NZ of diseases found on californian thistle.

"The team collected samples from hundreds of farms throughout NZ and found several pathogens of particular interest."Dr Bourdot said one of these, the vascular wilt fungus Verticillium dahliae, a pathogen that causes diseases in many crops, was common on the thistle in this survey."The fungus produces spores inside the thistle that are released by mowing, dispersed by splashing rain and then gain entry into other thistle plants through wounds."

To investigate, Beef + Lamb NZ funded an experiment on 12 farms throughout NZ over two years. The experiment showed that mowing in the rain produced a 30 per cent reduction in the ground cover of thistle in the spring, compared with mowing in dry conditions. For now, the biological basis of the wet mowing effect remains unproven. However, the research shows conclusively that mowing in the rain works to reduce californian thistle abundance. As Dr Bourdot said, "it is a simple technique that farmers can use right now at little cost".If more tests show the fungus is the reason behind the mowing-in- the-rain effect, it could potentially be formulated and marketed as a biological herbicide. Farmers could apply it when they mow paddocks in the rain to increase the effect.