A new tool will provide farmers and land managers more robust information on which to base more effective management of Californian thistle (Cirsium arvense).

For the first time, scientists at AgResearch have developed a model that simulates population growth of the thistle. It has just been published in the online publication, Ecology and Evolution.

Californian thistle was recently estimated to cost almost $700 million each year in lost agricultural productivity in New Zealand.

Based on substantial experimental data gathered by New Zealand and overseas scientists over many years, the model allows scientists to compare different defoliation strategies, whether that be through use of herbicide, mowing or biological control like the green thistle beetle (Cassida rubiginosa) which is currently established in several parts of New Zealand.

Principal Scientist Graeme Bourdôt says defoliation of the thistle is widely regarded as the most effective way to halt its population growth in a pasture.

“The amount of root that the thistle produces over the growing season is what regulates population growth. The more you defoliate the thistle, the less root it can produce.

“We’ve always known there would have to be a ‘tipping point’ because the root can only be produced if there is foliage above the ground. So the tipping point has to be where you defoliate enough so the plant cannot produce enough root to replace what was there during the current season. This model allows us to figure out where that tipping point is.”

Dr Bourdôt says the model shows that a single defoliation during the growing season (typically December - March) and repeated each year isn’t going to reduce the thistle, it will simply stabilise the population.

In comparison, the model shows that undertaking two treatments at specific times during the year will bring about population decline if repeated annually.

As an example, Dr Bourdôt says, the model shows that if a farmer chooses to mow the thistle firstly in December and then again in February each year, then the thistle population is likely to go into quite rapid decline, halving in density each year. Mowing at other times of the year is likely to be less effective, causing slower rates of decline.

This backs up past field experiments and provides good guidance for farmers across all grazing systems in choosing the best time to defoliate the thistle.

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