A long-term experiment was established to examine the crop yield and the weed infestation, focussed on Canada thistle (Cirsium arvense), as effect of different intensity of primary tillage (mouldboard plough deep or shallow, double-layer plough, chisel plough) in combination with or without stubble tillage. The most effective ways to keep the infestation of C. arvense at a low level were deep mouldboard ploughing and the use of a double-layer plough. After the experiment had run seven years, the thistle biomass was < 2 g DM m-2 in these treatments, compared to 23–26 g DM m-2 in the treatments with chisel ploughing or shallow ploughing. In all treatments, stubble tillage in addition to primary tillage significantly reduced the thistle biomass by 30–80 %. A high density of lucerne/grass re-growth occurred in the chisel plough treatment. The soil seed bank of thistles ranged between 220 (deep plough) and 6,400 seeds m-2 (chisel plough) in the sixth year of the experiment. Stubble tillage is essential if the chisel plough or shallow ploughing is used for conservation tillage in organic farming. The double-layer plough can control C. arvense comparable to deep ploughing
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