A new tool in the fight against Calis

“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.