AbstractData from twenty‐two comparisons carried out at ADAS Experimental Husbandary Farms are used to compare untreated and formic acid‐treated silages. Additive treatment led to an improved fermentation in some crops, particularly those of low DM concentration (<262 g kg‐1). Where this occurred there were associated benefits in silage digestibility (+0·234 units), intake (+16%) and the growth rate of young cattle (+0·28 kg d‐1). Where the fermentation of the untreated silage was good, both digestibility and animal performance associated with treated and untreated silages were similar. It is suggested that the justification for using formic acid in a commercial situation is thus restricted to occasions where the untreated crop would be liable to develop a clostridial fermentation. These may be when crops contain less than 35 g water‐soluble carboh