SAEVA Proceedings 2016 | Page 134

  Factors affecting the success of embryo recovery The most important factors affecting the likelihood of recovering an embryo are the timing of insemination and the intrinsic fertility of the donor mare and stallion (for reviews see Squires et al. 1999; Stout 2003); when AI is employed, stallion fertility, and therefore embryo recovery, is further influenced by semen dose, quality and method of preservation. In the case of young fertile mares inseminated with fresh semen from fertile stallions, more than 70% of flushes should yield an embryo (Losinno et al. 2001). However, embryo recovery rates drop dramatically when aged mares (>14 years) or mares with a history of sub-fertility are used as donors (Squires et al. 1982; Vogelsang and Vogelsang 1989: Meadows et al. 2000), or when chilled transported or, in particular, frozen-thawed semen are used (Meadows et al. 2000; Stout 2003). In Sporthorse practice in Europe and America, embryo recovery rates typically range between 30 and 50%, largely because owners often wish to recover embryos from aged donor mares inseminated with chilled or frozen semen (Squires et al. 2003). The time at which the flush is performed relative to ovulation also influences embryo recovery, primarily because the equine embryo does not enter the uterus until day 6 or 7 after ovulation (Battut et al. 1997, 2001); flushing on day 6 is therefore associated with a significantly lower embryo recovery rate (Boyle et al. 1989) simply because the embryo has yet to exit the oviduct. As will be discussed later, the equine embryo’s unusually late time of uterine entry is problematic when trying to recover pre-expansion embryos for cryopreservation, not least because the exact time of embryo descent may also be influenced by season (delayed early in the breeding season), mare age (delayed in older mares: Squires et al. 1999; Meadows et al. 2000)) and type of semen (delayed when frozen semen is used). Multiple ovulation Typically, mares ovulate only a single follicle per oestrus, however some mares will double or triple ovulate spontaneously and, in some breeds (e.g. Thoroughbreds and Warmbloods), rates of spontaneous multiple ovulation may exceed 30% (Davies Morel et al. 2005). In the context of ET, spontaneous multiple ovulation is useful because it is associated with an increase in total embryo yield and the number of pregnancies in recipients (Squires et al. 1987; Losinno et al. 2001), although there are indications that the number of embryos per ovulated follicle may be reduced if the follicles ovulate from the same ovary (Riera et al. 2005). Nevertheless, when rates of spontaneous multiple ovulation within an ET donor population are high (e.g. 30%: Losinno et al. 2001) the efficiency of the programme as a whole increases markedly. For similar reasons, the commercial expansion of bovine ET was aided enormously by the early development of cheap and effective protocols for superovulation, based on equine chorionic gonadotrophin (eCG) or pituitary extracted follicle stimulating hormone (FSH), that resulted in embryo yields averaging 5 per flush and peaking at 50 in occasional cows (Kafi and McGowan 1997). In marked contrast, the failure to develop a reliable protocol for pharmacologically inducing multiple ovulation in mares was a major reason 15-­‐18  February  2016      East  London  Convention  Centre,  East  London,  South  Africa     133