244 IN VITRO EMBRYO PRODUCTION WITH CONVENTIONAL OR Y-SEXED SEMEN IN BEEF CATTLE

Abstract

An experiment was designed to evaluate in vitro embryo production following the use of frozen-thawed conventional or Y-sexed semen from a single Brangus and a single Braford bull of proven fertility. Semen was obtained by splitting the same ejaculate to be frozen directly or sex-sorted and then frozen. Oocytes were obtained from 69 ultrasound-guided follicle aspiration (ovum pickup) sessions performed at random stages of the oestrous cycle without superstimulation in 24 Brangus and 10 Braford cows and heifers. Viable oocytes (n = 1120) were matured in TCM-199 medium with NaHCO₃ and supplemented with 1% fetal bovine serum. Frozen-thawed sperm from the Brangus and Braford bulls were selected with Percoll for IVF, capacitated in Fert Medium, and used at a final concentration of sperm per milliliter for conventional (non-sexed) semen and 2 × 10⁶ sperm mL^–1 for Y-sexed semen. After 16 h (sexed) or 18 h (conventional) of co-incubation with oocytes in Fert Medium, presumptive zygotes were denuded and cultured in SOF supplemented with 0.4% BSA under oil at 37°C, 5% CO₂, and saturated humidity for 7 days. The total number of oocytes matured and fertilized from the Brangus donors was 538 and 318 for conventional and sexed semen, respectively. The total numbers of oocytes matured and fertilized from the Braford donors were 139 and 125 for conventional and sexed semen, respectively. Data were compared by ANOVA for mixed models, using breed and type of semen as fixed variables and cow (i.d.) as a random variable. Cleavage and blastocyst rates were first transformed by square root and then analysed by ANOVA for mixed models. Mean (± s.e.m.) number of total viable oocytes collected, cleaved zygotes, and blastocysts produced per ovum pickup session did not differ (P = 0.18) between breeds (Brangus: 17.1 ± 1.6, 10.0 ± 0.9, and 6.2 ± 0.7 v. Braford: 13.9 ± 2.8, 7.6 ± 1.5, and 4.0 ± 0.8), and there was no breed × semen interaction on the mean number of cleaved zygotes and blastocysts produced. However, the mean (± s.e.m.) number of cleaved zygotes and blastocysts produced was significantly higher (P < 0.05) when the oocytes were fertilized with conventional semen (10.7 ± 1.2 and 6.5 ± 0.8) than with sexed semen (7.7 ± 0.7 and 4.3 ± 0.6). The mean cleavage rate was also significantly higher (P < 0.05) when the oocytes were fertilized with conventional semen (76.8 ± 3.9) than with sexed semen (54.1 ± 4.2). Blastocyst rate tended to be higher (P = 0.1) with conventional semen (40.5 ± 3.3) than with sexed semen (33.6 ± 4.2). Although in vitro production may be the preferred alternative for the production of embryos of a known sex, the number of blastocysts produced might be reduced as compared with the use of non-sexed semen from the same bull.