AbstractPerennial ryegrass, harvested as second‐cut material on 10 and 11 July 1990, was treated with either formic acid at 31 t‐1or an acid‐salt type additive at 61 t‐1and ensiled in roofed 150 t bunker silos. Subsequently both silages underwent a predominantly lactic fermentation. Nevertheless the acid‐salt‐treated silage had a significantly higher quantity of formic acid (19vs12 g kg DM‐1) and significantly lower levels of lactic (98vs118 g kg DM‐1) and acetic acid (11vs17 g kg DM‐1) compared with formic acid‐treated silage. In‐silo losses and effluent production were similar.Each silage was individually fed to 10 October‐calving Friesian dairy cows (average weight 565 kg) from weeks 2 to 15 of lactation, together with 3 kg d‐1of a compound feed containing 190 g kg DM‐1crude protein and with an estimated metabolizable energy content of 12·6 MJ kg DM‐1. The acid‐salt additive had no significant effect on silage DM intake, daily milk yield, milk protein or cow liveweight change, but significantly increased milk butterfat content compared with formic acid‐treated silage.It is concluded that the acid‐salt type additive produced little difference in terms of either silage fermentation or animal performanc