124 Analysis of the factors affecting the pregnancy rate in cows implanted with transferred Zebu cattle embryos produced in vitro

To evaluate the influence of externals factors, 1087 embryo transfer (ET) procedures on cows, from many genetic groups, maintained in an intensive grazing system with native grasses, the ET was performed with Zebu cattle embryos produced in vitro between March 2022 and April 2023. Embryo transfer technologies were devised with the purpose of fostering the reproduction of genetically valuable animal; however, there are multiple factors, both intrinsic and extrinsic, affecting their success rate when using embryos produced in vitro, extracted from donor Zebu cows by ovum pickup (OPU). Climatic conditions at levels outside the resting state of the animal frame the productivity of the individual, destabilising the comfort conditions and subjecting it to alterations of physiological functioning that leads it to an environment of heat stress. The extrinsic factors analysed in the donor Zebu cows were: the livestock production unit in which they were bred (n = 13), number of transfers performed in the unit (1 transfer, >1 transfer), region/geographical area (n = 4), and season in which the transfer was done (rainy or dry; spring, summer, fall or winter); and the intrinsic factors were: breed of the cows (Guzerat, Gyr, Sardo Negro, Brahman, Nelore, Indo-Brazilian), semen (normal or sexed), breed of the bulls (Dutch, Gyr, Sardo Negro, Brahman, Guzerat, Nelore), developmental stage of the transferred embryo (early blastocyst, expanded blastocyst, hatching blastocyst, hatched blastocyst) and embryo quality (excellent or good). The factors analysed in the receiving cows were the size of the corpus luteum (CL1, CL2, and CL3), and the season of the year (dry or rainy; spring, summer, fall or winter) in which the transfer was performed. The statistical analyses were done with the STATISTICA v10 software, ci² was used for the analysis of the main effects, and a multivariate grouping analysis was performed to estimate the correlation between factors. The general pregnancy rate was 37.99 ± 1.47, although there were differences (P < 0.05) observed among the production units (in a range of 2.78–75%), and when certain environmental factors, such as the season of the year and geographical area where the OPU was carried out, were varied; that was not the case for the frequency of embryo transfers. Regarding the intrinsic factors, the breed of the donors did not have any effect (P > 0.05) on pregnancy rate, which was the lowest in Sardo Negro specimens (33.89 ± 2.17%) and the highest in Gyr, Brahman and Nelore specimens (>40%). The type of semen, bull breed and developmental stage of the transferred embryo did not cause any variations on the pregnancy rate (P > 0.05); only the quality of the embryo showed any effects (P < 0.05), either excellent (38.87 ± 1.53) or good (26.32 ± 50.8). In the case of the receiving cows, the season of the year showed an effect (P < 0.05), but not the size of the corpus luteum (P > 0.05). The extrinsic factor (variable) grouping analysis showed a correlation (r > 0.90) between the season of the year in which the OPU was performed and pregnancy rate, while embryo quality, type of semen, and the embryo developmental stage were the intrinsic factors that showed the highest correlation with it (r > 0.80). It is concluded that the environmental factors surrounding the ovum pickup, and the quality of the transferred embryo, are to be considered when developing Zebu embryo transfer programs in humid tropical environments.