ALS-resistant prickly sow thistles have already been identified in Canada, France, Norway, and the USA, with resistance to a range of sulfonylurea actives and imazamox.
Glasshouse tests showed the UK population is highly resistant at 30g/ha metsulfuron-methly (4 times recommended field rate) (Figure 2 and 3), as well as resistant to field rates of thifensulfuron-methly and imazamox.
Fortunately, at the moment the resistant prickly sow thistle can still be controlled using other herbicides (e.g. mecorprop-P). However, as with all weeds, an integrated weed management approach should be used to increase control and reduce the risk of resistance. Effective non-chemical methods of control for prickly sow thistle include soil cultivation (ploughing – high; shallow cultivation ~5cm depth – medium) and using stale seedbeds.