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peer-reviewed Intensively managed grasslands are globally prominent ecosystems. We investigated whether experimental increases in plant diversity in intensively managed grassland communities can increase their resource use efficiency. This work consisted of a coordinated, continental-scale 33-site experiment. The core design was 30 plots, representing 15 grassland communities at two seeding densities. The 15 communities were comprised of four monocultures (two grasses and two legumes) and 11 four-species mixtures that varied in the relative abundance of the four species at sowing. There were 1028 plots in the core experiment, with another 572 plots sown for additional treatments. Sites agreed a protocol and employed the same experimental methods with certain plot management factors, such as seeding rates and number of cuts, determined by local practice. The four species used at a site depended on geographical location, but the species were chosen according to four functional traits: a fast-establishing grass, a slow-establishing persistent grass, a fast-establishing legume, and a slow-establishing persistent legume. As the objective was to maximize yield for intensive grassland production, the species chosen were all high-yielding agronomic species. The data set contains species-specific biomass measurements (yield per species and of weeds) for all harvests for up to four years at 33 sites. Samples of harvested vegetation were also analyzed for forage quality at 26 sites. Analyses showed that the yield of the mixtures exceeded that of the average monoculture in >97% of comparisons. Mixture biomass also exceeded that of the best monoculture (transgressive overyielding) at about 60% of sites. There was also a positive relationship between the diversity of the communities and aboveground biomass that was consistent across sites and persisted for three years. Weed invasion in mixtures was very much less than that in monocultures. These data should be of interest to ecologists studying relationships between diversity and ecosystem function and to agronomists interested in sustainable intensification. The large spatial scale of the sites provides opportunity for analyses across spatial (and temporal) scales. The database can also complement existing databases and meta-analyses on biodiversity–ecosystem function relationships in natural communities by focusing on those same relationships within intensively managed agricultural grasslands. The Agrodiversity experiment was a coordinated continental-scale field experiment. The coordination of the network was supported by EU COST Action 852: Quality Legume-Based Forage Systems for Contrasting Environments. COST852 provided funding for regular meetings and for scientific networking between partners. The collation of this database was supported by the Irish Research Council for Science, Engineering and Technology through a research fellowship to L. Kirwan, with additional support from Science Foundation Ireland Research Frontiers Programme (09/RFP/EOB2546).