RESUMO
The objectives of the present study were to: 1) compare the reproductive efficiency of embryo transfer (ET) recipients after synchronization of estrus or a 5-day synchronization of ovulation protocol for fixed time ET (FTET), and 2) determine the effect of reutilization of intravaginal P4 devices (CIDRs), up to four times, in a 5-day FTET protocol. In Experiment 1, 817 dairy heifers were assigned to one of three groups: PGF + estrus detection, 5-d FTET protocol with new (1.38 g P4) or 2nd use CIDR (previously used once for 5 d). Fresh in vitro produced embryos were transferred 7 ± 1 day after estrus (PGF + estrus) or GnRH (5-day FTET). Utilization rate (transferred/treated) was greater (P < 0.001) in heifers submitted to FTET compared to ET after estrus, however pregnancies per ET (P/ET) were not different (P > 0.10). As a result, pregnancy per treated (P/treated) recipient was greater (P < 0.05) in heifers in the 5-day FTET protocol. In Experiment 2, 40 dairy heifers without a corpus luteum (CL) were randomly allocated into one of four groups using new, 2nd use, 3rd use (previously used twice for 5 d/each), or 4th use (previously used thrice for 5 d/each) CIDRs. Circulating P4 was reduced (P < 0.01) with each reutilization. In Experiment 3, ovarian follicular dynamics were evaluated in 238 dairy heifers submitted to a 5-day protocol with either new, 2nd use, 3rd use or 4th use CIDRs at random stages of the estrous cycle. Prostaglandin F2α (PGF) was administered at CIDR removal and again 24 h later. Ovulation was induced by GnRH treatment 72 h after CIDR removal. Preovulatory follicle diameter increased (P < 0.001) progressively with increasing CIDR reutilization. Ovulation rate did not differ between treatments, however, interval from CIDR removal to ovulation decreased (P < 0.001) in heifers receiving 3rd and 4th use CIDRs compared to new or 2nd use. Finally, in Experiments 4 and 5, 1203 heifers submitted to a 5-day FTET protocol were randomly assigned to receive either a new CIDR, a 3rd use CIDR (Experiment 4) or a 4th use CIDR (Experiment 5). Despite the increase in CL volume on D5 in heifers treated with 3rd use (P = 0.03) or 4th use CIDRs (P < 0.01), there were no differences (P > 0.05) in utilization rate, P/ET, or P/treated. Thus, use of a 5-day FTET synchronization protocol improves reproductive efficiency by increasing recipient utilization, and reutilization of CIDRs up to four times in recipient dairy heifers does not compromise reproductive performance.