The extensive protein degradation occurring during ensiling decreases the nutritive value of silages, but this might be counteracted
by tannins. Therefore, silages from two legume species containing condensed tannins (CT) – sainfoin (SF) and birdsfoot trefoil (two
cultivars: birdsfoot trefoil, cv. Bull (BTB) and birdsfoot trefoil, cv. Polom) – were compared for their in vitro ruminal fermentation
characteristics. The effect of combining them with two CT-free legume silages (lucerne (LU) and red clover (RC)) was also
determined. The supply of duodenally utilisable CP (uCP) in the forages was emphasised. The legumes were each harvested from
three field sites. After 24 h of wilting on the field, the legumes were ensiled in laboratory silos for 86 days. Proximate constituents,
silage fermentation characteristics, CT content and CP fractions were determined. Subsequently, silage samples and 1 : 1 mixtures
of the CT-containing and CT-free silages were incubated for 24 h in batch cultures using ruminal fluid and buffer (1 : 2, v/v). Each
treatment was replicated six times in six runs. The effects on pH, ammonia and volatile fatty acid concentrations, protozoal counts,
and total gas and methane production were determined. uCP content was calculated by considering the CP in the silage and the
ammonia in the incubation fluid from treatments and blanks. Statistical evaluation compared data from single plants alone and
together with that from the mixtures. Among treatments, SF silage contained the least CP and the most CT. The non-protein
nitrogen content was lower, favouring neutral detergent soluble and insoluble protein fractions, in the SF and RC silages. Absolute
uCP content was lowest in SF and SF mixtures, although the ratio to total CP was the highest. In comparison with LU, the
ammonia concentration of the incubation fluid was lower for SF, RC and BTB and for the mixture of SF with LU. The total gas and
methane production was similar among the treatments, and the total volatile fatty acid production was decreased with the
CT-containing legumes. Protozoal count was increased with the mixtures containing LU and either SF or BTB compared with single
LU. In conclusion, compared with the other legumes, SF and RC have similar advantages as they show limited proteolysis during
ensiling. In addition, SF supplies more uCP relative to total CP. The CT-containing legumes also differed in their effect on ruminal
fermentation and ammonia formation, probably because of their different CT contents. Thus, SF and its mixtures appear promising
for improving the protein utilisation of ruminants.