Cover crop effects on the growth of perennial weeds in two long-term organic crop rotations

Summary

Cover crops are important in organic arable cropping systems because they improve soil fertility and suppress annual weeds in the post-harvest period, but their effectiveness against perennial weeds is less clear. This study analysed outbreaks of perennial weeds in two long-term organic crop rotations with and without cover crops in the period 2011-2022 in Denmark, and in particular examined the effects of cover crops. In addition, the impact of including forage plantain (Plantago lanceolata) in cover crop mixtures on the growth of perennial weeds was studied from 2020 to 2022. Plantain is known to contain secondary metabolites that inhibit nitrification and may influence nitrogen (N) availability. This study found that cover crops did not hinder outbreaks of Cirsium arvense, Sonchus arvensis, Elytrigia repens or other perennial weed species. E. repens actually proliferated more with cover crops whose presence prevented repeated mechanical post-harvest interventions. The crop rotation involving regular cutting of a one-year or two-year green manure crop provided the best management of perennial weeds if repeated post-harvest soil cultivation was conducted in two out of four or five years. This could only be done in the absence of cover crops. Mixing plantain into the cover crop mixtures did not change the outcome of competition between cover crops and perennial weeds in the post-harvest period. Instead, plantain significantly reduced the number of shoots emerging from perennial weeds in the subsequent growing season. The causality of this effect needs further elaboration in order to understand and utilise plantain as a potential source of weed suppression in combination with cover crops.