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

Perennial weeds can be a major constraint to organic crop production and direct control actions applied between crops can then be necessary to reduce the problems. We conducted two experiments, one on a sandy loam and one on a sandy soil in Denmark, with the aim of studying the efficacy of different implement types and strategies. The treatments were employed against mixed stands of perennials after harvest of spring barley in two consecutive years. Time of treatment, cultivation depth and combinations of implements constituted the strategies. Treatment effects were evaluated in the growing season that followed the post-harvest treatments. In the one experiment, repeated tine cultivation caused an 80–90% annual reduction of the population of mainly Cirsium arvense. With treatments conducted in two consecutive years, the accumulated effects reached 99% control. In the second experiment, power take-off driven implements with rotating weeding devices demonstrated similar control efficacies against a mixed stand composing C. arvense, Tussilago farfara, Elytrigia repens and Artemisia vulgaris. One pass was conducted a week after barley harvest followed by another pass 3 weeks later and ending the strategy with mouldboard ploughing in the succeeding spring. Grain yields did not differ among the treatments in the two experiments as a result of the generally high effectiveness exerted by the control strategies. Especially post-harvest control strategies based on rotating weed devices and mouldboard ploughing appear to be effective solutions against mixed stands of perennials on sandy soils but they do not comply with optimal nutrient management in organic cropping. Therefore, intensive autumn cultivation is only relevant where a perennial weed problem is uncontrollable by other means.