Reproductive seasonality influences oocyte retrieval and embryonic competence but not uterine receptivity in buffaloes.

Since buffaloes are a seasonal, polyestrous species, optimizing reproduction during the non-breeding season is a key factor in increasing the reproductive and productive efficiency of herds. Ovum pick-up associated with in vitro embryo production and embryo cryopreservation is an alternative to reduce seasonal impacts. We studied the effects of seasonality in buffalo oocyte donors and embryo recipients during the favorable and non-favorable breeding seasons. Donors were evaluated for oocyte recovery and blastocyst production rate as dFBS (donors in favorable breeding season) or dNBS (donors in non-favorable breeding season). Embryos produced from dFBS or dNBS were cryopreserved by vitrification or the slow-freeze method for direct transfer and transferred to recipients in the favorable (rFBS) or non-favorable breeding season (rNBS). The heifers or cows were subjected to a fixed-time embryo transfer protocol and conception rates were determined on day 30 and on day 60. The oocyte recovery was lower in dFBS than in dNBS (7.6 vs. 10.0 oocyte/OPU, p = 0.0262); while no difference was found comparing blastocyst production rate (23.7% vs. 30.9% of blastocysts, respectively). Embryos from dFBS resulted in greater (p = 0.0013) conception rates on day 30 compared to dNBS (46.5% vs. 22.4%, respectively), despite the breeding season. The rFBS and rNBS treatments had similar (p = 0.6714) conception rates on day 30 (38.0% vs. 33.0%, respectively), indicating similar uterine receptivity. However, heifers on FBS had higher (p = 0.0003) conception rates on day 30 than cows (73.9% vs. 13.3%, respectively) when receiving embryos from dFBS. Vitrification and direct transfer had similar (p = 0.1698) conception rates on day 30 (30.4% vs. 41.4%, respectively). In conclusion, in vitro-produced embryos derived from dFBS were more competent in establishing pregnancy than dNBS counterparts, independent of recipients' reproductive seasonality. Heifers achieved better conception rates than cows during the favorable breeding season when the embryo came from dFBS. Cryopreserved in vitro produced embryos represent a reliable alternative to reduce seasonal variations in buffalo reproduction. The data elucidate the seasonal effects on embryo competence and on recipients' uterine receptivity, affording new strategies to implement ovum pick-up associated with in vitro embryo production programs in buffalo herds.

Embryo transfer in buffalo (Bubalus bubalis).

The use of assisted reproductive technologies, such as superovulation and in vivo embryo production and in vitro embryo production (IVEP), has increased rapidly in recent years and is now applied worldwide for genetic improvement in beef and dairy buffaloes. Although in vivo embryo production has been shown to be feasible in buffalo, low efficiency and limited commercial application has been documented. These results could be associated with low antral follicle populations, high levels of follicular atresia and/or failures of the oocyte to enter the oviduct after superovulation. Additionally, IVEP technology has been shown to be an important tool for multiplying genetic material from donors of superior merit, and promising results have been achieved with the use of ovum pick-up (OPU) along with IVEP in buffalo. However, several factors appear to be critical for successful OPU/IVEP, including circulating levels of anti-Müllerian hormone, antral follicle populations, sizes of the follicles available for the OPU, reproductive seasonality, semen (sire) used for IVEP, donor category and farm. Furthermore, technologies applied to control follicular wave emergence and ovulation at predetermined times, without the need for estrus detection in recipients, has facilitated management and improved the efficiency of embryo transfer programs in buffalo herds. Conclusively, with the considerable evidence of poor results with in vivo embryo production in buffaloes, the association of OPU with IVEP represents a new alternative for the exploitation of buffalo genetics.