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Equine embryo transfer was first described by Japanese scientists in 1972, with successful embryo survival achieved by transvaginal ultrasound-guided injection of the embryo into the uterus of recipient mares.1,2 Earlier investigators had produced a mule and a hinny by surgical embryo transfer between horses and donkeys.3 The development of equine embryo transfer lagged behind the cattle industry, where embryo transfer in cattle was developed 20 years previously and had been commercially available for more than 10 years by the early 1970s.4
The commercialisation of equine embryo transfer was much slower as superovulation was not achieved, breed associations were slow to approve the registration of embryo transfer-derived foals, and surgical transfers were usually performed.3 While successful transcervical equine embryo transfers were reported in 1975,5 others reported lower pregnancy rates using a transcervical technique compared to a surgical technique.6,7 Therefore, initially equine embryos were largely transferred via a surgical approach through the flank,4 or ventral midline8 to expose a uterine horn, a practice that was widely used in cattle for embryo transfer.9 Other techniques that were used to transfer embryos included transcervical techniques using: insemination pipettes, cassou guns, disposable embryo transfer guns, Wilsher forceps-assisted transfers and transvaginal ultrasound-guided uterine injection.
A variety of factors were identified as important for the survival of the transferred equine embryo. These included: embryo quality, embryo size, degree of synchronicity of ovulation …
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