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Factors affecting pregnancy rates in beef embryo recipients
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The main objective of implementing embryo transfer in beef operations is to accelerate the rate of genetic progress in the herd. Among the main factors that affect the use of these technologies are related to nutrition, management and estrus synchronization. Although prostaglandin F2α (PGF2α) has been used most commonly for synchronization of estrus, the requirement for estrus detection and the variability in the interval from treatment to estrus and ovulation has adversely affected its performance in embryo transfer programs.
To avoid limitations associated with estrus detection, treatments that synchronize the time of ovulation, which were developed originally for fixed-time AI, have been utilized f/or FTET. These treatments are generally divided into those that are GnRH-based and those that are estradiol-based. In either case, the recipient protocols include the insertion of a progesterone (P4) releasing device for 5 to 8 days, depending on the protocol.
Recent studies with GnRH-based protocols have suggested that reducing the length of exposure of the P4-releasing device insertion to 5 days and increasing the interval from P4-device removal to GnRH and fixed-timed AI to 3 days may improve pregnancy per AI (P/AI) as compared to the traditional 7-day GnRH/P4 device protocol in beef cattle. Furthermore, it was suggested that a reduction in the length of the growth phase of the ovulatory follicle prior to ovulation, as occurs in some animals treated with the conventional 7-day protocols, alters the steroidogenic capacity of the dominant follicle prior to ovulation and the resulting CL, and decreases the ability of the uterus to support embryo development. Similar pregnancy rates per embryo transfer (P/ET) were obtained with in vitro-produced embryos to those of recipients synchronized with two PGF2α treatments 14 days apart and estrus detection and those synchronized using a modified 5-day Co-Synch+CIDR protocol (no GnRH at P4 device insertion, PGF2α at P4 re- moval on Day 5 and GnRH on Day 8).
Based on these findings, we evaluated the effectiveness of an estradiol/P4 treatment protocol in which the exposure to P4 device was reduced to 6 days and proestrus was lengthened by the administration of GnRH 72 h after P4 device removal instead of ECP at de- vice removal. The protocol for FTAI was named J-Synch. This treatment protocol has resulted in higher P/AI rates in beef heifers compared to the conventional protocol in which the P4-device is removed on Day 7 and ECP is given at that time. Furthermore, in a series of experiments that were conducted recently to evaluate the performance of the J-Synch protocol in embryo transfer programs the P/ET rate was greater in the J-Synch (49.4%) than in the conventional synchronization pro- tocol (41.0%; P<0.05).
Although the previously described protocols have performed adequately for several years, recent attention has been directed to the effect of estrus expression and estradiol concentrations during growth of the preovulatory follicle on embryo growth and pregnancy. In recipients showing estrus, we have shown a significantly (P<0.05) greater P/ET (48.3% vs 30.1%) and lower pregnancy losses (25,6% vs 66.7%) than in recipients not showing estrus. Therefore, use of tail-paint or estrus detection patches in recipients would help identify animals showing estrus by simply running them through the chute at the appropriate time, without the necessity of labor intensive estrus observations. These modifications can be easily implemented in recipient synchronization programs and should result in overall higher pregnancy rates.
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IRAC, Argentina
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