The aim of the study was to determine the effects of different proportions of Medicago media Pers. in mixtures with Dactylis glomerata L. on the yield of aboveground biomass, protein yield and relative chlorophyll content in orchard grass leaves. The study was carried out in 2011-2013. A strict field experiment was conducted in the Educational and Research Station of the University of Warmia and Mazury in Olsztyn, Poland (53o72' N; 20o42' E), on mineral soil of quality class IVa and very good rye complex. Pure-sown orchard grass and orchard grass in mixtures with hybrid alfalfa were investigated. The experimental factor was a varied proportion (30%, 50%, 70%) of alfalfa seeds in the mixtures. Results demonstrated that the proportion of alfalfa in mixtures increased gradually in subsequent years, but its share in the aboveground biomass was disproportionate to the percentage of sown seeds. Alfalfa introduced as a component of the mixtures increased the relative chlorophyll content in orchard grass leaves, but no significant differences were found between plots with 30% and 70% proportions of alfalfa. The mixtures gave significantly higher yields when compared to pure-sown orchard grass. Depending on the proportion of alfalfa in the mixtures, the yield was about 18 to 27% higher and more reliable in the study years. The introduction of alfalfa to the mixtures caused a significant 56% increase in protein yield per area unit as compared to pure-sown orchard grass. A highly significant correlation coefficient shows that the leaf greenness index can be regarded as an important prediction rate of plant yield.

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