Proteins from forage legumes are often poorly utilized by ruminants. Indeed, they are too rapidly degraded in the rumen to be efficiently absorbed in the intestine. However, the condensed tannins, secondary metabolites produced by some species such as bird’s-foot trefoil, reduce protein degradation. This study consisted in the assessment of the addition of tannins to forage samples of lucerne or white clover. The addition, in different proportions, of bird’s-foot trefoil forage to lucerne or to white clover resulted in mixtures containing from 0 to 5.2% tannins. A decrease in in vitro protein solubility was observed. The addition of purified tannins extracted from bird’s-foot trefoil and incorporated at rates of 0 to 5% to lucerne samples also induced a reduction in protein solubility. These results need validation with trials using animals, before management techniques or breeding objectives can be proposed for these species.