Strategies to control Canada thistle (Cirsium arvense) under organic farming conditions

Three strategies for controlling Cirsium arvense including (i) repeated stubble tillage with subsequent forage crop cultivation, (ii) repeated mowing of a ryegrass-clover ley and (iii) forage crop cultivation following a ryegrass-clover ley ploughed in May/June were investigated in field experiments over 3 years at the Experimental Farm for Organic Agriculture 'Wiesengut' in North-Rhine Westphalia, Germany. The development of C arvense (shoot density, shoot size and ground cover) was regularly assessed on fixed standardized subplots. In the medium-term (9 months), repeated stubble tillage (i) decreased shoot density and regrowth capacity of C. arvense more effectively than a mowed ryegrass-clover ley (ii and iii). However, after 22 months, strategies (i) and (ii) resulted in a similar strong reduction of C. arvense shoot density of 95 and 97%, respectively. At this time, the efficacy of strategy (iii) (89%) was not significantly different to that of strategies (i) and (ii). After 26 months, the effect of all strategies was still apparent; however, the efficacy of strategy (iii) was significantly lower than that of strategy (ii). Generally, the different strategies showed only minor differences, thus delivering options for optimal strategies of thistle control under given specific conditions of sites and cropping systems.