The aim of this study was to assess the effect of sward enrichment in valuable grass and legume species through undersowing on the amount and quality of yield and its usefulness for ensilage. Two-factorial field experiment in a cross system was set up on experimental meadow. Three 0.8 ha plots were delineated, one was undersown with a mixture of grasses and Trifolium pratense L. (MI), the second - with a mixture of grasses and Lotus corniculatus L. (MII), and the third (control) was not undersown. The second analysed factor included the form of fertilisers: mineral NPK versus manure fertilisation. Botanical composition of sward, yielding and the content of nutritive components were estimated every year. Mown sward of the first cut was ensiled in large cylindrical bales. The content of nutritive components and the concentration of fermentation products was analysed in silage. Applied undersowing resulted in the increased share of legumes at the cost of grasses and weeds. Undersowing was also the factor significantly differentiating biomass yield. The highest yields in 2014 were obtained from plots undersown with a mixture containing Trifolium pratense L. In the second study year the biggest yield was noted for sward undersown with a mixture containing Lotus corniculatus L. Sward from undersown plots had higher content of total protein, lower content of sugars and significantly lower sugar to protein ratio. The enrichment of botanical composition of sward through undersowing exerted a significant impact on nutritive value of silages. The greatest nutritive value (expressed as relative feed value RFV) was found in silages made of sward undersown with a mixture of grasses and Lotus corniculatus L.