Fifty-five multiparous Holstein-Friesian cows were used in a 20-week continuous design study to determine the effect of maize silage maturity on food intake and milk production. Forage maize (cv. Hudson) was harvested and ensiled at target dry matter (DM) contents of 230 (T23), 280 (T28), 330 (T33) and 380 (T38) g per kg fresh weight (FW). The mean values for volatile-corrected DM (VCDM), starch, neutral-detergent fibre (NDF), crude protein and predicted metabolizable energy (ME) content of the four maize silages were 226, 290, 302 and 390 g/kg FW, 114, 274, 309 and 354 g/kg VCDM, 574, 447, 431 and 448 g/kg VCDM, 96, 80, 74 and 75 g/kg VCDM and 10·3, 11·5, 11·6 and 11·2 MJ/kg DM, respectively. Grass silage containing 296 g VCDM per kg FW was produced from the primary growth of a perennial ryegrass sward. At week 3 of lactation cows were allocated to one of five forage treatments offered ad libitum. The forage treatments were either grass silage alone (TGS) or a 3 : 1 DM ratio of maize and grass silage designated as T23, T28, T33 and T38. All cows also received 8·7 kg DM per day of a dairy concentrate. Forage VCDM intake for TGS was lower (P < 0·001) than for T23 to T38. Increasing maize silage DM content from 226 (T23) to 290 (T28) g/kg increased (P < 0·05) forage VCDM intake from 10·9 to 13·3 kg/day but a further increase to 390 (T38) g/kg tended to reduce VCDM intake. When compared with TGS, the inclusion of maize silage increased milk yield from 28·0 kg/day to 29·4, 32·7, 33·0 and 30·8 kg/day for T23 to T38, respectively, the increases being significant for T28 to T38. However, milk yield was reduced (P < 0·05) when the DM content of the maize silage increased from 302 to 390 g/kg. Increasing maize silage DM content from T23 to T33 reduced (P < 0·05) milk fat content from 45·8 to 41·8 g/kg, which was also lower (P < 0·05) than for TGS. The inclusion of maize silage increased fat yield with a significant difference (P < 0·05) between TGS and T28. The inclusion of maize silage increased milk protein content (P < 0·05) and protein yield (P < 0·001) when compared with TGS. While increasing maize silage maturity did not affect (P > 0·05) milk protein content, protein yield was higher (P < 0·05) for the two intermediate DM contents. There were no treatment effects on body condition score. It is concluded that the changes in composition of maize silage with increasing maturity, which are associated with increased starch and reduced NDF content, resulted in large increases in food intake and yield of milk and protein as crops matured from T23 to T33. However, when crop maturity increased further to T38 there was a tendency for DM intake and yield of milk and protein to decline.