Use of intravaginal progesterone releasing device and estradiol 17 β is equivalent to estradiol 17 β and long acting progesterone in synchronizing acyclic embryo surrogate mares

Long-acting progesterone (LA P4 ) is the most common hormone used to synchronize acyclic surrogate mares for embryo transfer (ET). However, mares synchronized with LA P 4 not used for ET for various reasons (e.g. donor yielding a negative flush or nonviable embryo) do not respond to a resynchronization in a short time, likely due to residual LA P 4 in plasma. This results in economic losses to ET programs due to days that the surrogate mare is not being utilized. Intravaginal progesterone releasing devices (IPRD) for cattle have been used in transitional mares to hasten cyclicity. However, IPRD has not been tested to timely synchronize ET surrogate mares. This study aimed to assess serum progesterone (P4 ) concentrations, pregnancy rates after ET and pregnancy losses of embryo surrogate mares synchronized with IPRD. In Experiment 1, crossbred acyclic mares (n = 12) received estradiol 17β  ([17 βeta, Botupharma, Brazil] 10, 20 and 10 mg, respectively, on 3 consecutive days. At 24 hours after the last estradiol injection, an IPRD (Sincrogest, Ouro Fino, Brazil) containing 1 g of natural P 4, was inserted vaginally and kept for 9 days. Blood samples were collected daily for 11 consecutive days and P4  concentrations assessed using RIA. Experiment 2 was conducted for 2 breeding seasons (2016 - 2018), with crossbred embryo surrogate mares, randomly assigned to 3 groups: 1) cyclic mares (n = 43) having ovulation confirmed after induction (follicle ≥ 35 mm) with histrelin acetate (250 µg, IM, Strelin, Botupharma); 2) acyclic mares (n = 57) treated with estradiol 17 β for 3 days (as above) and then given a single dose of LA P 4 (P4-300, 1.5 g Botupharma); or 3) acyclic mares (n = 57) treated with estradiol 17β  for 3 days and then given an IPRD (Sincrogest). Day-8 embryos were transferred 4 - 8 days after ovulation or P 4 treatments, using a nonsurgical, transcervical technique. Mares in Group 3 had IPRD removed immediately before ET, and immediately after ET, a new IPRD was inserted. Pregnancy diagnosis was performed at 5 days after ET, and 30 and 60 days later. Once pregnancy was first confirmed, mares in Groups 2 and 3 received weekly injections of LA P 4 (1.5 g) until 120 days of pregnancy. Mares in Group 3 had the device removed 3 days after pregnancy diagnosis. Data on P4  concentrations were analyzed using ANOVA-RM and Tukey posthoc and data on pregnancy rates and losses were compared by multivariate logistic regression (p < 0.05). In Experiment 1, P 4 concentrations significantly increased from IPRD insertion (0.5 ± 0.2 ng/ml) to next day (15.8 ± 3.1 ng/ml), and then maintained satisfactory P 4 concentrations deemed suitable for successful establishment of pregnancy until day 9 (7.7 ± 2.3 ng/ml). After IPRD removal, P 4 concentrations were reduced significantly to baseline (0.78 ± 0.4 and 0.67± 0.3 ng/ml on days 10 and 11, respectively). For Groups 1-3, there was no difference in pregnancy rates (64, 66, and 69%, respectively) or pregnancy losses by 60 days of pregnancy (17, 13, and 10%). In conclusion, IPRD used herein resulted in a rapid increase in P 4 and sharp decline, upon its insertion and removal, respectively. In addition, this device can be used as a compatible alternative to LA P 4 to synchronize acyclic embryo surrogate mares.

Keywords: Equine, recipient mare, embryo transfer, fertility, hormonal therapy

This manuscript was originally published in the journal Clinical Theriogenology Vol 12(3) Sept 2020.  Clinical Theriogenology is the official journal of the Society for Theriogenology (SFT) and the American College of Theriogenologists (ACT).  This content has been reproduced on the IVIS website with the explicit permission of the SFT/ACT.