Abstract: Sainfoin (Onobrychis viciifolia) is a tanniniferous, leguminous plant that has potentially beneficial effects on protein utilization in ruminants. Since ensiling causes protein breakdown and elevated levels of buffer soluble N (BSN), we studied the distribution of N before and after ensiling sainfoin. Three varieties of sainfoin were either direct-cut and frozen directly or wilted and frozen before later ensiling in mini-silos with and without acidification with Promyr (PM; an acidifying commercial mixture of propionic and formic acid) and with or without polyethylene glycol (PEG). Extractable tannins (ET) and protein bound tannins (PBT) were measured with an HCl/butanol method in an attempt to correlate tannin levels to N fractions. The sainfoin silages showed good ensiling characteristics and had relatively high concentrations of un-degraded protein. The effect of wilting on BSN levels (g/kg N) was dependent on sainfoin variety (P<0.001). PEG increased and PM decreased the level of BSN in the silages (P<0.001). PM treatment also produced less non-protein N and ammonia-N (P<0.05) as compared with no additive. Addition of PEG to the silage increased the BSN-proportion 1.8- and 2.6-fold for both DM stages. A strong tannin-protein binding effect is, therefore, confirmed in sainfoin. However, correlations between tannin levels (ET and PBT) and BSN were poor in the (non-PEG) silages, indicating either that the HCl/butanol method is unsuitable for measuring tannin in silages or that qualitative attributes of tannins are more relevant than quantitative. The HCl/butanol method seems therefore not to be useful to predict degradation of protein in sainfoin silages.