The effect of bale density (low: 122–127 kg dry matter (DM) · m–3 vs high: 189–190 kg DM · m–3) and the addition of formic acid (5 l · t–1) on the in situ DM and crude protein (CP) degradation of wilted lucerne (Medicago sativa L.), red clover (Trifolium pratense L.) and red fescue (Festuca rubra) silages was determined. The effective DM degradability of lucerne, red clover and red fescue silages reached 59.8%, 52.8% and 51.9%, respectively, that of CP, 69.5%, 55.8% and 58.4%, respectively. High bale density decreased (P < 0.05) only DM degradability (53.9% vs 49.5%) and the a-value of red clover silage. The addition of formic acid had no effect on the DM degradability of silages. Bale density and formic acid, and their interaction, exerted the most significant effect on the effective degradation of CP and the content of rapidly degradable fraction a of lucerne silage. The effective degradation of DM and CP of red fescue silage was not affected by the experimental factors.