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12 Assessment of oocyte and embryo production in senescent beef cows

Fertility associated with age has been poorly documented in cattle. There is evidence that more than 50% of cows became infertile (failure to bear a calf for two successive years) at the age of 15. The causes of infertility in cows have been attributed to poor oocyte quality and/or follicle depletion. The present study aimed to compare the rates of oocyte and in vitro embryo production of fertile cows older than 15 years. Sixteen Nellore cows (Bos taurus indicus) 200.3 ± 4.9 months old were submitted to two ovum pickup series (OPU1 and OPU2) at a 2-year (2017 and 2019) interval. Before the OPU1 of each series, the cows were prepared with an IM injection of 2 mg estradiol benzoate and 0.15 mg prostaglandin F2α analogue and a progesterone-releasing intravaginal device (1 g). After five days, the progesterone-releasing intravaginal device was withdrawn and the OPU1 was performed. The second OPU (OPU2) was performed 5 days later to aspirate only the growing follicles. The number and morphologic quality of recovered cumulus-oocyte complexes were registered. After IVM (tissue culture medium 199) and fertilization (Tyrode's solution) of cumulus-oocyte complexes, the presumptive zygotes were cultured in synthetic oviductal fluid with amino acids to assess developmental rates to blastocyst. Data were analysed by chi-squared and Wilcoxon test. The number of oocytes recovered at OPU1 was higher in 2107 than in 2019 (27.8 ± 6.6 vs. 10.8 ± 4.2; P < 0.01), whereas in OPU2 the production of oocytes was not different (14.7 ± 3.3 vs. 10.9 ± 3.1). In OPU1, eight cows in 2017 and four in 2019 produced more than 20 oocytes, compared with only one cow in OPU2 of both years (P = 0.02). Although there was no significant difference in the proportion of viable oocytes in OPU1 (83.6% and 87.2%) and OPU2 (85.5 and 68.0%), more viable oocytes were produced at OPU1 of 2017 (23.2 ± 5.8) compared with OPU1 of 2019 (9.4 ± 4.1), OPU2 of 2017 (12.6 ± 2.7) and OPU2 of 2019 (7.4 ± 2.4), respectively (P < 0.05). There was no significant difference in the proportion of cleaved oocytes in OPU1 (74.2 and 75.1%) and OPU2 (68.9 and 79.8%). Two cows in 2017 (12.5%) and five in 2019 (31.2%) did not produce embryos in both OPUs. Cows produced more embryos (blastocysts) in OPU1 at 2017 (7.3 ± 1.6) than 2019 (2.8 ± 0.6; P < 0.05), whereas in OPU2 there was no difference (4.1 ± 0.8 and 3.1 ± 0.8). In conclusion, senescent fertile cows decreases the number of oocytes and embryos as they approach 20 years of age; however, the quality of oocytes does not appear to be compromised. Financial support was provided by FAPESP (proc. 2018/07450-5).