The influence of individual features and the breed of donor heifers on the efficiency of oocyte retrieval by ovum pick-up

The oocytes’ retrieval from lived cows (Ovum Pick-Up, OPU) is the most important element in the system of in vitro production (IVP) of cow embryos. In this regard, increasing the efficiency of OPU is a key factor for the widespread implementation of IVP technology in cattle breeding. The present work shows for the first time the possibility of obtaining IVP embryos from OPU oocytes obtained from a local breed of Yaroslavl cows. In Yaroslavl breed comparing to Simmental breed, the variability of the number of follicles was observed both between individual animals and between sessions in the same animals. We did not observe significant differences in the recovery rate and qualitative characteristics of the obtained cumulus-oocyte complexes, which allows using standardized protocols for donor heifers of both Simmental and Yaroslavl breeds. The aim of our work was to study the influence of individual features and breed characteristics of donor heifers on the efficiency of oocyte production by OPU. The studies were carried out in the laboratory of experimental embryology in 2020-2022 on mature clinically healthy heifers of the Simmental and Yaroslavl breeds aged of 17 to 36 months. Oocytes were obtained by transvaginal ultrasound-controlled puncture of follicles with an interval between sessions of 7 days according to the methodical guidelines using systems Bovine OPU (Minitube GmbH, Germany). In the first experiment, the efficiency of OPU was studied in individual donor heifers of the Simmental ( n = 7; 50 sessions) and Yaroslavl breeds ( n = 5; 25 sessions). In the second experiment, a comparative study of the OPU efficiency in donor heifers of the Simmental ( n = 6; 12 sessions) and Yaroslavl breeds ( n = 5; 25 sessions) was carried out using OPU parameters optimized for animals of the Simmental breed. The OPU efficiency was evaluated using the following criteria: the number of ultrasound-visible follicles, the number of retrieved cumulus-oocyte complexes (COCs), the degree of oocyte retrieval, the ratio of oocytes suitable for production of IVP embryos from the total number of derived oocytes. We observed high variability of the average number of ultrasound-visible follicles among individual donor heifers, both of Simmental (4.71-11.50 follicles; Cv = 47.9 %) and Yaroslavl breeds (5.80-9.80 follicles, Cv = 32.0 %), while the differences between some donors within breeds were highly significant (p £ 0.001). Differences in the number of ultrasound-visible follicles led to differences in the number of derived COCs among individual donors, both of Simmental (2.33-5.17 COCs) and Yaroslavl breeds (3.60-6.00 COCs). Comparative studies of donor heifers of the Simmental and Yaroslavl breeds did not show significant differences in the average number of ultrasound-visible follicles (7.33±0.62 vs. 6.96±0.45), the number of obtained oocytes (4.17±0.69 vs. 3.36±0.41) and the number of suitable oocytes (3.08±0.60 vs. 2.52±0.29). Thus, a high variability in the average number of ultrasound-visible follicles and the number of obtained oocytes between individual donor heifers of the Simmental and Yaroslavl breeds was established. Considering the high positive correlation between the number of aspirated follicles and the number of retrieved oocytes ( r = 0.97 and 0.72 for donor heifers of the Simmental and Yaroslavl breeds, respectively), it is advisable to select animals characterized by a large number of ultrasound-visible follicles to increase the efficiency of OPU. The absence of significant differences related to quantitative and qualitative characteristics of the retrieved oocytes allows us to recommend the OPU parameters optimized for donor heifers of the Simmental breed to be applied in heifers of the Yaroslavl breed without a noticeable loss of efficiency.