Thesis (M.S., Animal and Veterinary Science) -- University of Idaho, 2015 | Hypotheses were that pre-synchronization of estrus in dairy heifers, in comparison with no pre-synchronization, would 1) reduce days to first AI, and 2) result in an increased proportion of heifers pregnant within the first week of the breeding program. The 14 d CIDR group (n=119) received a controlled internal drug releasing insert (CIDR) on d -30, which was removed on d -16, followed by an injection of prostaglandin F2? (PG) on d 0. The 2X PG group (n=118) received an initial injection of PG on d -11, and a second injection of PG on d 0. The control group (1X PG; n=121) received an injection of PG on d 0. Heifers received AI upon detected estrus. Pre-synchronization affected (P<0.05) days to first AI and the proportion of heifers pregnant within the first week of breeding eligibility. The 14 d CIDR treated heifers had reduced days on feed (P<0.05) (breeding pen entry to projected calving date) compared to control heifers. .