Like CT, PPO allows the formation of complexes with proteins via the oxidation of phenolic compounds intohighly reactive quinones. At the end of the in vitro rumen fermentation, the effect of treatments on the NH3:total N ratiowas near significance with the tendency for a lower ratio in pure legume than in pure grass silages. More strikingly, theNH3:insoluble N ratio appeared to be generally lower for the treatments including legumes than for pure T. This resultsuggests that the remaining undegraded or poorly degraded proteins in silages containing the bioactive compounds werelikely still protected during rumen fermentation. In addition, the negative quadratics effects detected between T and SF onsome minor VFA which originate mostly from deamination of some amino acids, suggest that protein of T may have beenprotected by CT of SF when the two plants are mixed, as already reported previously with fresh cocksfoot and SF (Niderkornet al., 2012). An in vivo study could help verify if this implies a greater flow of proteins to the duodenum.