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The present study was designed to compare the efficiency of bovine embryo production for cryo- preservation between oocytes collected from donors in the growth phase of follicular development (GPFD) and those recovered from donors in the undefined phase (UPFD). Cyclic cows, Czech Siemental or Holstein dairy breeds, 4-6 years of age, slaughtered at the local abbatoir were used. They were divided into two groups based on ovarian morphology: I. GPFD donors with ovaries corresponding to the growth phase of the first follicular wave (estrus cycle days 3-4; n = 52), and II. UPFD donors with ovaries in any other phase of follicular development (undefined estrus cycle days; n = 89). A total of 3 771 oocytes were collected and 1 134 embryos were prepared as two separate populations by standard protocol. In total 352 excellent or good quality embryos at the early, advanced or expanded blastocyst stage from both donor groups were pooled and used for cryotolerance assessment. They were frozen on day 7 (D7) or day 8 (D8) after fertilization by the standard procedure. After thawing, the embryos were cultured for 72 h to the hatching stage. The percentages of both D7 embryos and advanced blastocysts were significantly higher (P ≤ 0.01) for oocytes collected from GPFD donors than for oocytes collected from UPFD donors (33.7 vs 28.6% and 43.0 vs 29.5%, respectively). The percentages of excellent or good quality embryos obtained from both D7 embryos and fertilized oocytes were significantly higher (P ≤ 0.01) for oocytes collected from GPFD donors than for oocytes collected from UPFD donors (63.6 vs 49.4% and 21.4 vs 14.1%, respectively). The post-thaw survival rates were significantly higher (P ≤ 0.01) for D7 than D8 embryos (80.4 vs 66.3%). In rela- tion to the developmental stage, the development and hatching rates were significantly higher (P ≤ 0.01) for D7 than D8 early blastocysts (75.0 vs 41.2% and 50.0 vs 5.9%, respectively) and for D7 than D8 advanced blastocysts, (73.7 vs 57.1 and 52.6 vs 28.6%, respectively). No differences were found between D7 and D8 expanded blastocysts after freezing-thawing. In conclusion, the collection of oocytes from donors in the growth phase positively influ- enced the in vitro production of bovine embryos for cryopreservation. The development of embryos produced with oocytes from GPFD donor group was accelerated and more excellent and good advanced blastocysts with greater cryotolerance were produced on day 7 after fertilization, as compared to embryos produced with oocytes from the UPFD donor group.

Collection of oocytes from donors in the growth phase of follicular development can enhance the production of bovine embryos for cryopreservation