179 Oocyte recovery and maturation rates in Estonian sport horses

Ovum pickup (OPU) and intracytoplasmic sperm injection (ICSI) are rapidly gaining interest in the horse industry. The ability to preserve the genetics, efficiently use scarce or expensive semen, and produce embryos out of the breeding season from competing mares has proven attractive to sport horse breeders. Recently, our research group introduced the commercial equine in vitro embryo production system in Estonia, achieving the first commercial ICSI pregnancy. The Estonian Sport Horse breed is a mix of European warm-bloods whose success in show jumping and dressage disciplines has gained international interest. Breed has been shown to significantly affect the efficiency of an OPU-ICSI program (Fonte et al. 2024 Theriogenology 223, 47–52). Thus, the aim of our study was to to evaluate the effect of the seasonality on oocyte recovery rates and the effect of different oocyte holding media on maturation rates in ESH. A total of 15 ESH mares, aged between 3 and 23 years, were subjected to OPU over the course of a year. The OPU procedure was performed as previously described by Papas et al. (2021 Animals 11, 2004), with prewarmed commercial equine OPU medium (Stroebech Media ApS). Recovered cumulus–oocyte complexes (COCs) were held at 22°C for 20 h in one of three holding media: an in-house made medium (IH) consisting of 40% M199 with Earle’s salts, 40% M199 with Hanks’ salts, 25 mM HEPES, and 20% FBS; a commercial equine oocyte medium (ST, Stroebech Media); or EmCare embryo holding medium (EM, ICPBio). Then, oocytes were matured in groups of 5–20 COCs in 300 µL of maturation medium (Equine IVM Medium; Stroebech Media ApS) under heavy oil (Stroebech Media Aps) for 28–32 h at 38.2°C under 5% CO₂ in air. Analysis of variance test was used to assess statistical differences. A total of 47 OPU sessions were performed, 649 follicles aspirated, and 425 (65.49%) COCs recovered. On average, 13.81 ± 4.88 follicles (mean ± SD) were aspirated and 9.04 ± 4.38 COCs retrieved per mare. A total of 305 oocytes (71.76%) exhibited the first polar body after cumulus cell removal. The nuclear maturation rate was significantly higher in oocytes held in IH medium compared with those held in ST medium (76.74% vs. 64.94%; P = 0.029). No significant differences were found between ST and EM holding media (64.94% vs. 73.74%; P = 0.123), nor between IH and EM media (76.74% vs. 73.74%; P = 0.321). No statistical differences were found in oocyte recovery rates based on the time of year in Estonia: winter, 66.06%; spring, 53.76%; summer, 72.59%; and autumn, 63.52% (P = 0.284). In conclusion, oocyte recovery and maturation rates in the Estonian Sport Horse breed are suitable for a commercial OPU-ICSI program. Moreover, overnight holding of oocytes in IH, EM, and ST holding media support good oocyte maturation rates in this breed. Despite the extreme weather conditions in Estonia (with an average air temperature of −6.3°C in winter and +17°C in summer), OPU efficiency remains stable across seasons.

This study was supported by the European Union through the European Regional Development Fund, grant number 616221790109.