The objective of the experiment was to compare follicular dynamics, ovulatory response to GnRH, and synchrony of estrus and ovulation among estrous-cycling and prepubertal beef heifers synchronized with a controlled internal drug-release (CIDR)- based or GnRH-PGF(2alpha) (PG) protocol. Estrous-cycling beef heifers were randomly assigned to 1 of 4 treatments (C1, C2, C3, C4), and prepubertal beef heifers were randomly assigned to 1 of 2 treatments (P1, P2) by age and BW. Blood samples were taken 10 and 1 d before treatment to confirm estrous cyclicity status (progesterone or =0.5 ng/mL estrous cycling). The CIDR Select (C1, n = 12; P1, n = 14)-treated heifers received a CIDR insert (1.38 g of progesterone) from d 0 to 14, GnRH (100 microg, i.m.) on d 23, and PG (25 mg, i.m.) on d 30. Select Synch + CIDR (C2, n = 12; P2, n = 11)-treated heifers received a CIDR insert and GnRH on d 23 and PG at CIDR removal on d 30. The CIDR-PG (C3, n = 12)-treated heifers received a CIDR insert on d 23 and PG at CIDR removal on d 30. Select Synch (C4, n = 12)-treated heifers received GnRH on d 23 and PG on d 30. HeatWatch transmitters were fitted at CIDR removal (C1, C2, C3, P1, and P2) or at GnRH administration (C4) for estrus detection. Ultrasound was used to determine the response to GnRH and the timing of ovulation after estrus. Among the estrous-cycling heifers, ovulatory response to GnRH and estrous response did not differ (P 0.05). Among the prepubertal heifers, more (P = 0.02) P1 heifers responded to GnRH than P2 heifers, but estrous response did not differ (P 0.05). Among the estrous-cycling heifers, variance for interval to estrus after PG was reduced (P 0.05) for C1 compared with each of the other treatments, and C2 was reduced (P 0.05) compared with C3. Variance for interval to ovulation after PG was reduced (P 0.05) for C1 compared with each of the other treatments. Among the prepubertal heifers, there was no difference (P 0.05) in variance for interval to estrus or ovulation. Results from C1 and P1 (T1) and C2 and P2 (T2) were combined to compare T1 and T2 among mixed groups of estrous-cycling and prepubertal heifers. Response to GnRH was greater (P 0.01; 81 T1 and 39 T2), and variances for interval to estrus and ovulation for T1 were reduced (P 0.01) compared with T2. In summary, CIDR Select improved (P 0.01) the synchrony of estrus and ovulation compared with Select Synch + CIDR.
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