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
