AbstractOne hundred and thirteen Holstein Friesian heifers (PIN (95) £88 s.d. 11.1) were used in a study to determine the effect of rearing regime, in terms of diet offered and target calving weight, on milk production in high genetic merit heifers. Eighty of the heifers were supplied from 11 commercial farms, the remainder were supplied from the herd at the Agricultural Research Institute of Northern Ireland. The heifers commenced the experiment at 7 weeks of age when they were allocated on the basis of source, live weight and genetic merit to one of four rearing regimes. The target weights at calving were 540 kg (Treatment 1) and 620 kg (treatments 2, 3 and 4). Treatments 1 heifers were offered grass silage-based diets during the winter and grass–based diets during the summer. Treatment 2 heifers were offered the same forage base along with additional concentrate supplementation. Treatment 3 heifers were offered a straw/concentrate diet during the winter and grass-based diets during the summer. Treatment 4 heifers received the same diets as treatment 3, apart from the first summer period when they were housed and offered a straw/concentrate diet. The heifers were mated at 14 months of age and were returned to the 11 farms one month prior to calving. Heifers reared on treatment 1 weighed less had a lower withers height (P<0.001) and were of a lower condition score (P<0.001) before calving than heifers reared on the other treatments. During early lactation (3 months post-calving) heifers reared on treatment 1 lost less weight and condition score than the heifers reared on treatments 2, 3 and 4. Over the winter housing period (mean 140 days) the average milk yield was lower for heifers reared on treatment 1 compared with heifers reared on treatments 2 (P<0.05), 3 (P=0.08) and 4 (P<0.05). Milk protein content was higher in animals reared on treatment 1 compared with treatment 2 (P<0.01), 3 (P<0.05) and 4 (P=0.12). Overall milk fat plus protein yield was lower for heifers reared on treatment 1 compared with treatments 2 (P<0.05), 3 (P=0.06) and 4 (P<0.05).