Functional diversity enhances grassland productivity, but species identity drives performance

Despite growing interest in increasing functional diversity for grassland management, the roles of species identity, traits, and interactions in enhancing productivity and sustainability of intensively managed grasslands remain underexplored. In a 2-year plot-scale field experiment, simulating slurry-based intensive dairy grasslands, thirty-nine grass communities comprising six species from three functional groups (stolon formation, high shoot density, deep rooting) were evaluated using a simplex mixture design to determine their influence on yield, composition, forage quality, and Poa trivialis suppression under moderate and high fertilization treatments (250 and 500 kg N ha⁻¹). Nitrogen (N) and potassium (K₂O) were applied in equal proportions to avoid K limitation. Multispecies mixtures produced 7–29 % higher aboveground biomass than average monoculture yield, with greater yield stability across harvests and experimental years. Overyielding was observed even in binary mixtures, while 5-species mixtures produced the highest yields and the lowest yield variance found in 4-species mixtures. Lolium hybridum (16.7 t ha⁻¹) and Dactylis glomerata (16.2 t ha⁻¹) contributed the highest to total community yields, while high N fertilization reduced diversity and promoted dominance of fast-growing grasses such as Lolium perenne and L. hybridum. However, mixtures comprising deep-rooted and fast-establishing species effectively suppressed P. trivialis , particularly when included at proportions of 30 % or higher. In contrast to the typical trade-off between biomass and nutritive value, this study found that multispecies swards had higher digestible organic matter and metabolizable energy content than monocultures across multiple harvests, underscoring the role of complementary functional traits in promoting productive and resilient grasslands. • Multispecies mixtures boosted yield by up to 29 % vs. monocultures. • Mixtures showed higher and more stable digestibility and energy content. • Deep-rooted, fast-growing species suppressed Poa trivialis effectively. • ≥ 30 % sowing of key species enhanced weed suppression in mixtures. • Functional diversity improves forage quality, weed control, and system resilience.