Production and reproduction of early and late bred dairy heifers.

A total of 253 heifers bred at first estrus after 350 d of age (350-d breeding age group) and 249 contemporary heifers bred at first estrus after 462 d of age (462-d breeding age group) were used to study the effects of age at first breeding on productive and reproductive performances of first lactation heifers. Heifers of both breeding age groups were subject to similar feeding and management practices. The average age at first calving was 698 d for the 350-d breeding group and 796 d for the 462-d breeding group. Although not statistically significant, heifers of the 462-d breeding group tended to have a higher conception rate at first service (47 vs. 38%) and fewer days between first service and conception (39 vs. 44 d) than those of the 350-d breeding age group. Breeding heifers as early as 350 d of age has no adverse effects on calving ease or retained placenta but does result in calves 1.2 kg lighter at birth. Heifers of the 350-d breeding group had lower milk, protein, and fat yields at both 168 and 308 d of first lactation than those of the 462-d breeding group. A 1-d reduction in age at first calving decreased 308-d milk yield by 2.01 kg for the 350-d breeding group as compared with 4.74 kg for the 462-d breeding group.