The aim of this study was to evaluate the effect of different proportions of hybrid alfalfa (Medicago media Pers.) grown in two-component mixtures with Festulolium (Festulolium braunii (K. Richt.) A. Camus) and orchard grass (Dactylis glomerata L.) on the macronutrient content of aboveground biomass. The study was conducted in 2011–2013 in Poland.  The experiment had a split-plot design with four replications, and the experimental variables were: (i) mixtures of Dactylis glomerata (Dg)+Medicago media (Mm), and Festulolium braunii (Fb)+Medicago media (Mm), and (ii) proportion of Medicago media seeds in the mixture: 30%, 50% and 70%. Pure-sown Dactylis glomerata and pure-sown Festulolium braunii were the control treatments. The proportion of hybrid alfalfa in the sward increased in successive years of the experiment. In the second and third year of the study, alfalfa was the dominant species in the mixture regardless of the proportion of sown seeds. In the analyzed grass mixtures, alfalfa was subjected to greater competitive pressure from orchard grass than from Festulolium. The choice of the grass component in mixtures with alfalfa exerted a greater influence on the macronutrient content of aboveground biomass than the proportion of alfalfa seeds in the mixture. Alfalfa and orchard grass mixtures accumulated more phosphorus (P) and potassium (K) than alfalfa and Festulolium mixtures. In turn, calcium (Ca) content was higher in alfalfa and Festulolium mixtures. The analyzed mixtures did not differ significantly in magnesium (Mg) levels. In comparison with pure-sown grasses, the cultivation of alfalfa-grass mixtures improved the K:(Ca+Mg) ratio which reached optimal values characteristic of high-quality fodder. The results of the study indicate that alfalfa-grass mixtures should be established based on the selection of a grass component adapted to local agroecological conditions, rather than the proportions of seeds in the mixture.