Sixteen first lactation dairy cows were used in a 2 × 2 × 2 × 2 factorial design experiment. Two silages, an extensively fermented inoculant-treated silage or a restricted fermented, mixed aliphatic carboxylic acid-treated silage were offered with two concentrate types (starch- or fibre-based) and supplementary lactic acid (LA, 0·4 kg/day) or soya oil (SO, 0·41 kg/day). Concentrates containing 180 g crude protein per kg were offered at 6 kg/day. Silage dry-matter intake tended to be higher (P = 0·07) with restricted fermented silage compared with the inoculant silage. Milk fat or protein concentrations were not significantly affected (P > 0·05) by silage fermentation type. LA inclusion had no significant effect (P > 0·05) on animal performance. SO inclusion significantly increased (P < 0-05) milk yield (+ 0·8 kg/day), decreased milk fat concentration and yield (40·2, 32·9 g/kg and 779, 667 g/dayfor - and + SO respectively, P< 0·001) and decreased milk protein concentration (-0·81 g/kg, P < 0·01). A significant concentrate × soya-oil interaction (P<0·01) suggested a greater reduction in milk fat concentration with the SO, starch compared with the SO, fibre combination. Modified acid-detergent fibre (P < 0·01) and neutraldetergent fibre (P < 0·001) apparent digestibility coefficients were significantly lower with starch- compared with fibre-based concentrates. Results suggest that milk production responses to changes in concentrate composition were not influenced by the extent of restriction of the silage fermentation process.