Direct control of perennial weeds between crops – implications for organic farming

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Perennial weeds can be a major constraint to organic crop production in several North European 26 countries. Preventive and cultural control measures may not suffice when infestations with perennials 27 become severe. Direct control actions applied between crops can then be necessary to reduce the 28 problems. We conducted two experiments, one on a sandy loam and one on a sandy soil, respectively, in 29 Denmark, with the aim of studying the efficacy of different implement types and strategies. The 30 treatments were employed against mixed stands of perennials after harvest of spring barley in two 31 consecutive years. Time of treatment, cultivation depth and combinations of implements constituted the 32 strategies. Treatment effects were evaluated in the growing season that followed the post-harvest 33 treatments. In one experiment, repeated tine cultivation caused an 80-90% annual reduction of the 34 population of mainly Cirsium arvense. With treatments conducted in two consecutive years, the 35 accumulated effects reached up to 99% control. In the second experiment, power take-off driven 36 implements with rotating weeding devices demonstrated similar control efficacy against a mixed stand 37 composing C. arvense, Tussilago farfara, Elytrigia repens and Artemisia vulgaris. One pass was 38 conducted a week after barley harvest followed by another pass 3 weeks later and ending the strategy with 39 mouldboard ploughing in the succeeding spring. Grain yields did not differ among the treatments in the 40 two experiments as a result of the generally high effectiveness exerted by the control strategies. 41 Especially post-harvest control strategies based on rotating weed devices and mouldboard ploughing 42 appear to be effective solutions against mixed stands of perennials but they do not comply with optimal 43 nutrient management in organic cropping. Therefore, intensive autumn cultivation is only relevant where 44 a perennial weed problem is uncontrollable by other means and should preferably be applied on limited 45 areas only.

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