The aim of the experiment was to determine the effect of applying an inoculant and formic acid at commercially recommended rates to big bale silage on the quality, protein and cell wall ruminal degradability and intestinal digestibility of undegraded protein. Wilted grass was ensiled with no additive, an inoculant (30 L/t) or formic acid (4 L/t). The composition of inoculated silage did not differ from that of the control silage. Formic acid supplementation decreased ammonia-N and lactic acid and increased acetic acid and water soluble carbohydrate content. The effective rumen degradability of silage N (determined in sacco) and the intestinal digestibility of undegraded protein (mobile bag) were not affected by the additive treatments. Cell wall degradation in the rumen was also not affected by inoculant or formic acid treatment.