AbstractA total of 1713 samples of silage from commercial farm silos were analysed to investigate the effect of dry matter (DM) content and chemical additives on fermentation as measured by ammonia‐N concentration and pH, Increasing DM content without additive use had a major beneficial influence upon fermentation. When silage DM contents were greater than 260 g kg‐183% of silages were well fermented, with average ammonia‐N concentrations of 94 g (kg total N)‐1and pH 4.36. With diminishing DM concentration the proportion of well fermented silages declined. In the DM range 220‐260 g kg‐167% of silages were well fermented with ammonia‐N concentrations of 125 g (kg total N)‐1and pH 4.30, in the DM range 180‐220 g kg‐148% were well fermented with ammonia‐N at 151 g (kg total N)‐1and pH 4.38 and with DM below 180 g kg‐1no silages were well fermented with ammonia‐N concentration of 252 g (kg total N)‐1and pH 4.84.The benefit of chemical additives, albeit at poorly defined and often inadequate rates, was small in comparison to that of increased DM concentration. Below DM concentrations of 180 and 220 g kg‐1, the only benefit was that formic acid decreased ammonia‐N to 151 g (kg total N)‐1and pH to 4.32 compared with 163 g (kg total N)‐1and pH 4.43 for untreated silages. Within the DM range 220‐260 g kg‐1formic acid decreased ammonia‐N level to 104 g (kg total N)‐1and pH to 4.07, and there was a slight benefit from using sulphuric acid/formalin which decreased ammonia‐N to 117 g (kg total N)‐1and pH to 4.23 compared with 125 g (kg total N)‐1and pH 4.27 for untreated silages. Above 260 g DM kg‐1both formic acid and sulphuric acid/formalin provided a small but consistent decrease in ammonia‐N and pH compared with untreated silages. Chemical additive use conferred no other benefit when compared with untreated silage. Calcium formate/sodium nitrite mixtures and acid mixture use provided no