The reduction of chemical inputs in agriculture is a current challenge. Perennial forage legumes such as lucerne produce protein-rich forage without synthetic nitrogen fertilizers but the crops may be invaded by weeds. Association of legumes with grasses are recognized to lower weed pressure but may alter forage yield and quality. Objective: Pure lucerne was compared to lucerne-grass mixtures, with the test of grass species differing for morphological, phenological and quality traits. The grass species were compared in mixture with lucerne for their performance, which was evaluated through weed occurrence, forage yield and quality. Methods: Seven perennial grass species were evaluated in association with lucerne and compared to pure lucerne and pure grass (one species only), in field plot experiments in two locations, without nitrogen fertilisation. Forage yield, botanical composition (lucerne, grass, weeds), protein and acid detergent fibre were measured in four cuts per year during three years. Treatments were compared, and mixture model effects such as direct and associated effects of the grass on dry matter yield of the species in the associations were calculated. Results: Lucerne-grass association reduced weed development compared to pure lucerne. The protein content was slightly lower in the associations than in pure lucerne but the association generally produced more protein per hectare than expected if the two species were grown in separated plots (i.e. protein overyielding). Depending on the grass species, the weed control, the forage quality, the proportion of lucerne and the dry matter yield of the association were differently affected. Species, with a small direct effect (on grass production) and a large associated effect (on lucerne production), such as timothy and meadow fescue, favoured lucerne proportion in the associations. Conclusions: Association of grass species with lucerne is a way to limit weed occurrence while maintaining protein content and forage yield in the association. These traits, as well as the lucerne proportion in the association, varied depending on the grass species. Implications: At a practical level, the application of herbicides on lucerne crops could be significantly lowered, and this with a limited impact on forage yield and quality. From a scientific point of view, the calculation of mixture model effects is of interest to analyse the outcome of species associations.