Cropping systems change to adapt to socio-economical and environmental constraints (e.g. simplified tillage) and to profit from technological innovations (e.g. genetically-modified crops). These changes can result in unexpected side-effects which are difficult to determine in fields. The objective of the present study was to use a model, FLORSYS, to evaluate such impacts ex ante on weeds. FLORSYS quantifies the effects of crop succession, management techniques and climate on multi-specific weed dynamics over the years; it was parameterized with functional relationships predicting model parameters (e.g. seed mortality) from species traits (e.g. seed coat thickness). Cropping systems typical of three French regions and various management modifications were simulated, using a weed flora consisting of seven mostly autumnal species and local weather sets. The Aquitaine scenario (maize monoculture with annual mouldboard ploughing) presented a total maximum weed infestation of approximately 0.01 plants/m² averaged over the simulation, consisting mostly of CAPBP, with a few VERHE and AVEFA. Weed flora in Burgundy (winter oilseed rape / winter wheat / winter barley, with ploughing before wheat) was more diverse (ALOMY, GERDI, STEME, a few CAPBP). Poitou-Charentes (rape / wheat / sunflower / wheat, with ploughing three years out of four) presented a 100-times higher infestation, consisting mostly of AVEFA and a few STEME. Modifying management practices changed both weed density and composition. For instance, simplifying or abandoning tillage greatly increased weed infestation, and no-till favoured grass weeds to the detriment of broad-leaved species. Infestations were related to species traits, and the selected traits depended on cropping systems. For instance, species with heavy seeds were selected in scenarios with frequent ploughing, probably because they can germinate and emerge even when deeply buried. The present methodology is now being adapted to evaluate the impact on biodiversity of crop management practices accompanying GM varieties in the EU project AMIGA.