Experiments were carried out to determine the effects of tannin content on N fractions and DM digestibility of birdsfoot trefoil harvested at 20% bloom stage. Five trefoil cultivars, harvested in both June and August, were used: Upstar, Mirabel, Empire, Leo, and Bt-8861. Forage was ensiled in 1.3-kg laboratory silos for 60 d with 4 silos per cultivar and harvest. Concentrations of DM and tannin in forage were determined prior to ensiling, and N fractions, DM digestibility, and pH in silage were also determined. Tannin concentration and protein N content of silage were lower in Empire than in the other cultivars. Ruminal degradability of DM and CP of Empire (low tannin) and Upstar (high tannin) birdsfoot trefoil were estimated with two fistulated cows using nylon bags that were incubated < or = 96 h. The DM-soluble fraction of Empire was significantly lower than that of Upstar for the first harvest only. This difference was partly due to the high proportions of dry shoots. The CP-soluble fraction of Empire was higher than that of Upstar, but the percentage of CP that was potentially degradable in the rumen was lower for Empire. This difference could result in different CP availability over time even for similar effective degradability of CP. Tannin had no additional effect on total CP degradability in the rumen.