246 INFLUENCE OF BREED AND SEASON ON IN VITRO EMBRYO PRODUCTION

Abstract

A retrospective analysis of in vitro production (IVP) data was done to determine the influence of breed and season on the production of viable oocytes and embryos. Cumulus‐oocyte complexes (COC) were obtained from 1946 ultrasound-guided follicle aspiration (ovum pickup) sessions performed at random stages of the oestrous cycle without superstimulation in Bos taurus and Bos indicus donors in commercial IVP in Argentina. Frozen-thawed conventional semen was used in beef cattle and conventional (n = 139) and sexed-selected (n = 481) semen in dairy cattle. The COC were classified, matured in B-199 medium, fertilized in IVF-SOF medium (Day 0), and cultured in SOF medium supplemented with 0.4% BSA under oil at 38.8°C, 5.5% CO₂, and saturated humidity for 7 days. The number of viable COC and transferable embryos in each breed and season were compared by ANOVA and means were compared by Fisher’s Least Significant Difference test. Proportions were first transformed by arcsin and then analysed by ANOVA. To simplify the interpretation of the results, breeds were grouped as follows: dairy Bos taurus (Holstein, n = 620), beef Bos taurus (Angus and Bonsmara, n = 229), Bos taurus × Bos indicus (Brangus and Braford, n = 1045), and Bos indicus (Brahman, n = 52). There was no interaction between breed and season for any of the end points analysed (P > 0.1). Mean (± standard error of the mean) numbers of viable COC and transferable embryos were higher (P < 0.01) in Bos indicus × Bos taurus (19.3 ± 0.4 and 5.3 ± 0.2, respectively) and Bos indicus (15.8 ± 1.4 and 6.8 ± 0.9, respectively) than in beef (11.6 ± 0.5 and 3.0 ± 0.2, respectively) and dairy (8.0 ± 0.2 and 1.6 ± 0.1, respectively) Bos taurus donors. Cleavage rates were higher (P < 0.01) in Bos indicus (72%) than in the other breeds (57% for Bos indicus × Bos taurus and dairy Bos taurus and 54% for beef). Transferable embryo rates were higher (P < 0.01) in Bos indicus (41%) and Bos indicus × Bos taurus (30%) than in beef Bos taurus (26%). Dairy Bos taurus had the lowest (P < 0.01) embryo rates of all breeds (21%). In dairy Bos taurus, cleavage rates, the number of embryos produced, and transferable embryo production rates were higher (P < 0.01) when conventional semen was used (62%, 2.8 ± 0.15, and 27%, respectively) compared to sexed-selected semen (55%, 1.3 ± 0.1, and 19%, respectively). With regards to season, the number of viable COC was highest (P < 0.01) in the spring (14.3 ± 0.5), lowest in the summer (11.3 ± 1.0), and intermediate in the fall (12.2 ± 1.2) and winter (13.7 ± 1.2), which did not differ. Although not affected significantly by season, the number of embryos produced was numerically lower in the summer (2.8 ± 0.4) than in the spring (4.2 ± 0.2), winter (4.5 ± 0.5), or fall (4.6 ± 0.5). In conclusion, in vitro embryo production was directly influenced by breed and season. Bos indicus influenced cattle and the spring season were preferable for commercial IVP programs that did not include superstimulation.