Incompatibility between fertility building measures and the management of perennial weeds in organic cropping systems

Fertility building measures are important components in improving the productivity of organic cropping systems without livestock. However, some measures seem to be incompatible with the control of perennial weeds that can have adverse effects such as significant weed competition. The influence of fertility building measures (N2-fixing crops in the crop sequence, cover crops and manuring) and the abundance of perennial weeds were studied in a long-term crop rotation experiment at two locations in Denmark. The aim was to gain insight into the factors that influence the growth of perennial weed species occurring in mixed stands. Data were obtained from three cycles of four-year arable crop rotations comprising various cash crops in rotations with and without annual whole-year grass-clover as green manure and subjected to four treatment combinations: with and without animal manure and with and without cover crops. Severe outbreaks of perennial weed problems did not occur at the location that had the highest soil fertility, whereas the other site demonstrated dynamic growth of Cirsium arvense and Elytrigia repens. Grain legumes tended to promote the growth of C. arvense, while manuring was neutral to C. arvense but beneficial to E. repens. Cover crops assisted the growth of E. repens since prolonged mechanical interventions were not possible. Compatibility was only achieved with grass-clover and C. arvense, meaning that green manure crops suitable for cutting and mulching could offer an important management option against C. arvense but not against E. repens.