New interest is given to alfalfa, an important forage legume crop, because it can contribute to agriculture sustainability. Mixtures of alfalfa with grass species combine, in a single sward, the advantages of the two species: seasonal growth pattern, biochemical composition, low nitrogen input. Forage production, but also the survival of each species, result from the ability of plants to get the resources needed for their growth. The present study aimed at describing alfalfa genetic variation for biomass production in different treatments: spaced plants, dense monocultures and alfalfa-tall fescue mixtures. Four harvests were taken in 2011 and 2012. In analyses of variance, the effects of alfalfa genotype and treatment on alfalfa biomass were highly significant. The interaction between genotype and treatment effects was significant, showing that genotype ranking for biomass depended on the treatment. These results will be complete with data on fescue biomass, plant height and nitrogen content to propose an alfalfa ideotype suitable for both monoculture and mixture.