Abstract
The objectives were to evaluate the effect of synchronization protocols on follicular development and estradiol 17-β (E2) and progesterone (P4) concentrations in dairy heifers. In experiment 1, 36 heifers were assigned to 1 of 6 synchronization protocols in a 3×2 factorial design: presynchronization with GnRH on study d −6 or −9 [study d 0=initiation of the Cosynch+CIDR (controlled internal drug releasing insert containing P4) protocol] or no presynchronization (control) and one injection of PGF2α or not on study d 0. In experiment 2, 126 heifers were assigned to 1 of 4 synchronization protocols in a 2×2 factorial arrangement: presynchronization or not with GnRH on study d −6 and injection of PGF2α or not on study d 0. In experiments 1 and 2, all heifers received a modified Cosynch protocol with CIDR for 7 d starting on study d 0. After the PGF2α of the Cosynch and removal of the CIDR, heifers were detected in estrus and inseminated. Those not inseminated by study d 10 received an injection of GnRH and were timed-inseminated. Ovaries were scanned by ultrasound on d 0, 2, and 5, daily from d 7 to 14, and on d 16. Blood samples collected on d 0, 2, 7, 9, and 16 were analyzed for P4, and the blood sample collected on d 9 was analyzed for E2. Pregnancy was diagnosed at 28 and 40±3 d after artificial insemination. In experiment 1, there was a tendency for the presynchronization protocol to affect the proportion of heifers ovulating in response to the first GnRH injection of the Cosynch+CIDR protocol. In experiment 2, a greater proportion of presynchronized heifers ovulated in response to the first GnRH injection. Although heifers receiving PGF2α had larger ovulatory follicles on d 7 and before ovulation and shorter intervals to estrus and ovulation, these heifers tended to have decreased concentrations of E2 during proestrus. Presynchronization of dairy heifers with GnRH increased ovulation in response to the first GnRH injection, and treatment of heifers with PGF2α at initiation of the Cosynch+CIDR protocol increased the size of the ovulatory follicle and reduced the intervals to estrus and ovulation.
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