Effects of rearing regime on body size, reproductive performance and milk production during the first lactation in high genetic merit dairy herd replacements

One hundred and thirteen high genetic merit Holstein-Friesian heifers were used in a study to determine the effect of rearing regime, in terms of diet offered and target calving weight, on body size, reproductive performance and 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). Treatment I 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 plus 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, except for the first summer period when they remained housed and were offered a straw/concentrate diet. The heifers were mated at 14 months of age and were returned to the 11 source farms one month prior to calving. Heifers reared on treatment I had a lower withers height (P < 0.001) and were of a lower condition score (P < 0.001) before calving than heifers reared on treatments 2, 3 and 4. During early lactation (3 months post calving) heifers reared on treatment 1 lost less weight and condition score than the heifers reared on the other treatments. Thus at the end of the first lactation live weights did not differ significantly between the treatments. However, body length remained shorter (P < 0.01) in treatment 1 compared with treatments 2, 3 and 4. First lactation milk yield (305 days) was lower for heifers reared on treatment 1 (7222 l) compared with heifers reared on treatment 2 (8020 l) (P < 0.01), 3 (7956 1) (P < 0.01) and 4 (79011) (P < 0.05). Similarly, milk fat plus protein yield was lower (P < 0.05) for heifers reared on treatment 1 (511 kg) compared with treatments 2 (544 kg), 3 (544 kg) and 4 (554 kg). The interval from calving to first recorded oestrus was shorter in treatment 1 compared with treatments 2 and 3 (P < 0.05). There was a tendency (P < 0.10) for heifers reared on treatment 1 to have a shorter calving interval (394 days) compared with treatments 2 (426 days), 3 (435 days) and 4 (458 days). In conclusion increasing the live weight of Holstein-Friesian heifers at first calving from 540 to 620 kg pre-calving increased milk yield proportionally by 0.11 but tended to increase the calving interval. Diet type during the rearing period had no effect on milk fat plus protein yield or reproductive performance.