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The success of invitro embryo production (IVEP) in horses has increased considerably during recent years, but little is known about the effect of the speed of invitro embryo development. Blastocysts (n=390) were produced by intracytoplasmic sperm injection of IVM oocytes from warmblood mares, cryopreserved, thawed and transferred into recipient mares on Days 3, 4, 5 or 6 after ovulation. The time required for invitro-produced (IVP) embryos to reach the blastocyst stage was recorded (Day 7 vs Day 8). The likelihood of foaling was affected by the speed of invitro embryo development and recipient day after ovulation at transfer. The odds ratio for foaling was ~0.63 for transfer of Day 8 (46%) compared with Day 7 (56%) IVP blastocysts. The highest likelihood of pregnancy (72%) and foaling (60%) was observed when IVP blastocysts were transferred to recipient mares on Day 4 after ovulation. Finally, the sex (colt:filly) ratio was higher after transfer of Day 7 (71%:29%) than Day 8 (54%:46%) IVP blastocysts, suggesting that the speed of embryo development is sex dependent. In conclusion, the speed of invitro embryo development in our IVEP system affects the likelihood of foaling and the sex of the foal.

The diameter of embryos recovered from mares on Day 8 after ovulation varies greatly, from as little as 130 μm to as much as 2500 μm. Several factors have been proposed to affect embryo size at recovery, one of which is the type of semen (frozen vs fresh or cooled-transported) used to inseminate the mare. In addition, it has been shown that smaller embryos ( 0.1) from that of recipients of small embryos from Groups 2 to 4 combined (8/15; 53.3%).

BACKGROUND: In vitro embryo production (IVEP) is increasingly popular but data assessing the outcome of transferred embryos are scarce. OBJECTIVES: To determine the likelihood of pregnancy and embryonic loss after transfer of frozen-thawed IVP embryos and identify factors influencing success. STUDY DESIGN: Retrospective clinical study. METHODS: Blastocysts (n = 261) were produced from immature oocytes of Warmblood mares (n = 116) by Intracytoplasmic Sperm Injection (ICSI) and in vitro culture, and cryopreserved. Thawed IVP embryos were transferred into recipient mares on day 4, 5 or 6 after ovulation. The influence of donor mare (age, reproductive history), recipient mare (age, reproductive status, management; in-house versus outpatient, day post-ovulation), embryo (interval from ICSI to blastocyst formation) and management factors (season when Ovum Pick-Up was performed, year and method of transfer) on likelihood of pregnancy and embryonic loss was examined, and the developmental stage of the IVP embryo at the time of transfer was estimated. RESULTS: The percentage of mares pregnant 7-10, 23 and 37 days after transfer was 56% (147/261), 49% (129/261), and 48% (124/261), respectively. Development of IVP embryos after transfer equated to day 5 or 6 in vivo embryos. With the exception of year of transfer, none of the factors had an impact on the likelihood of pregnancy or embryonic loss. Nevertheless, the likelihood of pregnancy tended to be lower for IVP embryos from infertile mares or when embryos were transferred into recipient mares on day 6 after ovulation rather than on day 4 or 5. Finally, the diameter of the embryonic vesicle 7 days post transfer was lower for pregnancies that were lost compared to those that were maintained. MAIN LIMITATIONS: Small sample size in some of the donor and recipient mare categories. CONCLUSIONS: Cryopreserved IVP embryos should be transferred into recipient mares on day 4 or 5 after ovulation and a slower rate of post-transfer vesicle expansion indicates a higher risk of subsequent embryonic loss. This article is protected by copyright. All rights reserved.

Equine embryos tolerate an unusually large degree of negative uterine asynchrony (recipient mare up to 5 days behind the donor mare). By contrast, positive asynchrony of more than 2 days results in a high incidence of early embryonic loss (EEL). Day 8 embryos range in diameter from approximately 130-1300 μm, with embryos smaller than 300 μm reported to suffer an increased incidence of EEL. However, it is not known whether this reduced viability is due to intrinsically poor embryo quality, or to inadvertent recipient uterine stage-embryo (positive) asynchrony. To examine whether small embryos survive better in Day 4-5 recipients than in recipients with a more advanced uterine stage, the likelihood of pregnancy (PR) and EEL for 62 small ( 0.1). The EEL for small embryos transferred into Day 4-5, 6-7 and 8-9 recipients was 20.8, 18.7 and 25.0%, respectively. We conclude that embryos recovered on Day 8 with a diameter

SummaryBackground Previous surveys reported a positive association between the length of the follicular phase and subsequent fertility in embryo transfer donor and Thoroughbred mares. However, it is unclear whether a longer oestrus positively influences fertilisation and oviductal development (oocyte quality, oviductal environment), or uterine receptivity and survival of the embryo in the uterus. Objectives To determine the effect of length of oestrus (characterised by duration of endometrial oedema) on likelihood of pregnancy and early embryo loss (EEL) in recipient mares after embryo transfer (ET). Study design Retrospective clinical study. Methods A total of 350 embryos recovered from 161 donor mares were transferred into 231 recipient mares during three consecutive breeding seasons. The following variables were analysed via two binary logistic regression models to determine their effect on pregnancy and EEL: 1) year of transfer, 2) season of transfer, 3) age of the recipient mare, 4) age of the donor mare, 5) operator performing the transfer, 6) singleton or twin embryo, 7) embryo size, 8) number of transfers to a given recipient in any one season, the use of 9) d-cloprostenol and 10) hCG in the recipient mare, 11) day of ovulation of the recipient mare at ET, 12) number of corpora lutea (CLs) at ET, and 13) duration of oestrus in the recipient mare. Results Age of the donor mare (P = 0.01), operator (P = 0.008), number of CLs at ET (P = 0.05) and the number of days of endometrial oedema during the oestrus preceding ET to the recipient mare (P = 0.004) influenced the likelihood of pregnancy. Early embryonic loss was influenced only by the year of transfer (P = 0.014). Main limitation Retrospective design of the study. The involvement of several veterinary surgeons over the 3-year period could have affected data recording. Conclusions The likelihood of pregnancy in recipient mares is positively correlated with the duration of endometrial oedema during the oestrus preceding ET. This suggests a role for an adequate duration of oestrogenic priming during oestrus on uterine receptivity and embryo survival. This article is protected by copyright. All rights reserved.