A coordinated continental-scale field experiment across 31 sites was used to compare the biomass yield of monocultures and four species mixtures associated with intensively managed agricultural grassland systems. To increase complementarity in resource use, each of the four species in the experimental design represented a distinct functional type derived from two levels of each of two functional traits, nitrogen acquisition (N2-fixing legume or nonfixing grass) crossed with temporal development (fast-establishing or temporally persistent). Relative abundances of the four functional types in mixtures were systematically varied at sowing to vary the evenness of the same four species in mixture communities at each site and sown at two levels of seed density. ; We thank many colleagues, too numerous to mention, for technical assistance with this manuscript. We thank Nyncke Hoekstra and three anonymous reviewers for their comments. Coordination of this project was supported by the EU Commission through COST Action 852 ‘Quality legume-based forage systems for contrasting environments’. L.K. was supported by a Research Fellowship from the Irish Research Council for Science, Engineering, and Technology and by an award from Science Foundation Ireland (09/RFP/ EOB2546). A contribution to the research leading to these results was conducted as part of the AnimalChange project (J.A.F, J.C., M.S. and A.L., grant agreement 266018) and the MULTISWARD project (O.H.-E., grant agreement FP7-244983), both of which received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013). The authors have no conflict of interest to declare.