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.

中文翻译：

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小母牛 5 天固定时间胚胎移植 (FTET) 方案的优化 I. 在 FTET 期间通过重复利用阴道内孕酮装置来控制循环孕酮

本研究的目的是：1) 比较发情同步或 5 天同步排卵方案的固定时间 ET (FTET) 后胚胎移植 (ET) 受者的生殖效率，以及 2) 确定胚胎移植 (ET) 受者的生殖效率。在为期 5 天的 FTET 协议中重复使用阴道内 P4 装置 (CIDR)，最多四次。在实验 1 中，817 头奶牛被分配到三组之一：PGF + 发情检测、5 天 FTET 协议与新的 (1.38 g P4) 或第二次使用 CIDR（以前使用一次 5 天）。在发情（PGF + 发情）或 GnRH（5 天 FTET）后 7 ± 1 天转移新鲜的体外产生的胚胎。与发情后 ET 相比，接受 FTET 的小母牛的利用率（转移/治疗）更高（P < 0.001），但是每个 ET 的妊娠率（P / ET）没有差异（P > 0.10）。因此，在 5 天 FTET 方案中，每个治疗 (P/治疗) 受者的妊娠率在小母牛中更高 (P < 0.05)。在实验 2 中，40 头没有黄体 (CL) 的奶牛被随机分配到四组中的一组，使用新的、第二次使用、第三次使用（以前使用两次，每次 5 天）或第四次使用（以前使用三次，每次 5 d/每个）CIDR。每次再利用都会减少循环 P4 (P < 0.01)。在实验 3 中，对 238 头奶牛的卵巢卵泡动力学进行了评估，这些小母牛在发情周期的随机阶段接受了新的、第二次使用、第三次使用或第四次使用 CIDR 的 5 天方案。前列腺素 F2α (PGF) 在去除 CIDR 时给药，24 小时后再次给药。在去除 CIDR 后 72 小时通过 GnRH 处理诱导排卵。随着 CIDR 再利用的增加，排卵前卵泡直径逐渐增加（P < 0.001）。排卵率在治疗之间没有差异，但是，与新的或第二次使用相比，接受第三次和第四次使用 CIDR 的小母牛从去除 CIDR 到排卵的间隔缩短（P < 0.001）。最后，在实验 4 和 5 中，1203 头接受 5 天 FTET 协议的小母牛被随机分配接受新的 CIDR、第 3 次使用 CIDR（实验 4）或第 4 次使用 CIDR（实验 5）。尽管第 3 次使用 (P = 0.03) 或第 4 次使用 CIDR (P < 0.01) 处理的小母牛在 D5 时 CL 体积增加，但利用率、P/ET 或 P/处理没有差异 (P > 0.05) . 因此，使用 5 天 FTET 同步协议通过增加受体利用率来提高繁殖效率，并且在受体乳小母牛中重复利用 CIDR 高达四倍不会影响繁殖性能。