First-cut wilted herbage (mainly Lolium perenne) was ensiled in barrels (120 l) either untreated (UT), treated with an inoculant, Pioneer 1188 (IN), or with a formic acid-based chemical additive, Foraform (FO). IN silage contained more fermentation products than FO, although applying the inoculant to wilted grass did not result in consistent changes in silage fermentation. Formic acid significantly decreased (P<0.05) the content of lactic and acetic acids and of ammonia-N, but increased the water soluble carbohydrates concentration as compared with UT and IN silages. Ruminal effective protein degradability was slightly but not significantly lower for FO silage (71.6%) than for UT (74.3%) and IN (74.0%) silages. Formic acid treatment significantly increased (P<0.05) intestinal digestibility of rumen-undegraded protein. Dry matter intake measured with sheep was not affected by treatments.