Prevention is essential to effectively manage weeds in grasslands. Yet, the identification of preventive measures can be tedious because numerous interacting management and environmental factors can influence grassland infestation by a weed. Case-control studies carried out on-farm allow a risk value to be ascribed to potential factors. We conducted a paired case-control study to assess the risk of the occurrence of Rumex obtusifolius in intensively managed permanent grasslands. Following a common protocol, in Switzerland, Slovenia and United Kingdom, parcels of land with high density of R. obtusifolius were compared with nearby parcels that had very few or no R. obtusifolius plants. Measured parameters included data about management practices, vegetation, and soil nutrients and texture. Analysed with multiple logistic regression, we showed that increased vegetation cover reduced the relative risk of R. obtusifolius occurrence while increased soil phosphorus and high soil bulk density raised the risk. Each of these effects was of comparable size across countries, as no interactions between country and any of the factors were observed. We conclude that case-control studies are a suitable tool to identify factors driving the infestation risk of grasslands regarding weeds such as R. obtusifolius. Results were achieved under the conditions of applied management, sometimes lasting for more than 10–20 years and allowed for direct recommendations to integrated weed management.