The use of forage legumes as a source of protein for ruminants is a sustainable strategy to reduce the use of inorganic-nitrogen fertiliser. In addition, some legumes species contain naturally bioactive secondary compounds, which could improve silage quality and digestive processes in ruminants. The aim of this study is to investigate the effects of bioactive legumes containing condensed tannins or polyphenol oxidase, ensiled alone or in mixture with a grass, on silage quality and conservation characteristics. Six mini-silos were prepared in triplicate as follows: 100% red clover (RC), 100% sainfoin (SF), 100% Timothy (T, control without bioactive compounds), binary mixtures 50% T + 50% RC, 50% T + 50% SF and ternary mixture 50% T + 25% RC + 25% SF. Condensed tannins remain unaltered during the silage-making process in terms of quantity and chemical structures, while polyphenol oxidase is activated during this process. All the silages that contained bioactive legumes were better conserved than the pure grass silo. In addition bioactive legumes were able to preserve protein from their degradation during the silage process. Inclusion of bioactive legumes can improve silage quality, and polyphenol oxidase may be more efficient than condensed tannins to improve the nitrogen value of silage.