The effect of formic acid on the fermentation was investigated in a 3 - factor experiment, in which the degree of prewilting, the storage temperature, and the amount of applied acid were considered. The material was obtained from are autumn cut, of young plant which is difficult to ensilage(DM 27.69, 44.0%). For experimental reasons the amount of acid applied 0.0, 0.25, 0.5 and 1.0% of green matters.