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Abstract The use of 4‐day CoSynch + Controlled internal drug release (CIDR) + timed artificial insemination (TAI) in dairy heifers has resulted in adequate pregnancy rates compared with the 5‐day CoSynch + CIDR + TAI protocol. The objective of this study was to compare follicular growth, timing of ovulation and serum progesterone (P4), estradiol (E2) and luteinizing hormone (LH) concentrations in dairy heifers treated with modified 4‐ or 5‐day CoSynch + CIDR protocols (CIDR for 4 or 5 days, PGF2α at CIDR removal and GnRH + TAI 72 h later). Twelve cycling Holstein heifers were randomly assigned to either the 4‐ or 5‐day Co‐Synch+CIDR (n = 6/treatment) to receive an intravaginal insert CIDR® containing 1.38 g of P4 for 4 or 5 days, respectively. At CIDR removal, 25 mg of PGF2α was injected IM; 72 h after CIDR removal, heifers received 100 μg of GnRH IM and timed artificial insemination (TAI). Follicular growth and timing of ovulation were assessed using transrectal ultrasonography. Blood samples were collected at the time of CIDR insertion and at frequent time points after CIDR removal for determination of P4 (at TAI), E2 (every 12 h) and LH (every 6 h during the first and second day and every 2 h on the third day). Heifers in the 4‐day group had smaller follicles from CIDR insert removal to ovulation compared with heifers in the 5‐day treatment. Five of six heifers (83.3%) in the 4‐day treatment ovulated at 90–96 h post CIDR insert removal, whereas most heifers in the 5‐day treatment (4/6; 66.6%) ovulated at 84–90 h post CIDR insert withdrawal. Heifers in the 5‐day treatment reached greater peak LH concentration between 48 and 72 h after CIDR insert removal and lesser E2 concentration at TAI than heifers in the 4‐day treatment. In conclusion, heifers in the 4‐day treatment had smaller follicular diameter at 0, 30, 36, 42 and 48 h after CIDR insert removal, longer interval from CIDR insert removal to ovulation, greater E2 concentrations at TAI, and lesser peak LH concentration than heifers in the 5‐day treatment. These results represent a baseline for further studies to determine if prolonging the interval to TAI by 6 h in the 4‐day CoSynch+CIDR would improve pregnancy risk.