Cultivation of forage mixtures offers several advantages over monocultures, but forage legumes like alfalfa (Medicago sativa L.) are mostly bred in pure stands. Our goal was to assess the extent of accession-by-cultivation system interaction when alfalfa plants are grown in pure stands or in an easily adaptable nursery system together with their companion grasses, thereby determining the system most suitable for selection in mixture. Spaced plants of 50 alfalfa accessions were grown on bare soil as control treatment (CONV), in a sown sward of short growing lawn cultivars of tall fescue (Festuca arundinacea Schreb.) and red fescue (F. rubra L.) (LAWN), and in a sown sward of taller forage cultivars of the same species (FORA). Accession-by-cultivation system interaction variances were large for growth habit but small for vigor. Phenotypic correlation coefficients (rp ) among the cultivation systems were high for vigor, whereby LAWN was somewhat more predictive for FORA (rp,FORA−LAWN = 0.83) than CONV (rp,FORA−CONV = 0.77). Observed accession-by-genotype interactions can be used pro or contra necessity for selection in mixture. However, the LAWN cultivation system might be a good compromise for practical breeding, allowing to account for given competition effects among species and to easily assess traits in the nursery.