The effects of heat stress on the population of the ovarian follicles and in vitro embryo production in dairy cows

This study aimed to evaluate the effects of heat stress (HS) on the follicular population at dominant follicle removal (DFR) and Ovum Pick-Up (OPU), oocyte quality and yields following OPU, and the subsequent in vitro embryo development in dairy cows. Ten Holstein cows were randomly assigned to either a heat stress challenge (HSC) or a control (CON) group (n = 5) in a split-plot crossover design with 2 periods. In Phase 1 (days 1-9), all the cows were fed ad libitum and were in thermoneutral conditions, and GnRH was given on Day 9. In Phase 2 (days 1-10), all cows were exposed to HSC using an electric heat blanket, while CON was pair-fed to HSC. On Day 7 (Phase 1), all the follicles were counted, and large follicles (>6 mm) were transvaginally aspirated (DFR). On Day 10, all the follicles were counted and transvaginally aspirated (OPU). After a 14-day washout period, the study was repeated (Period 2) and the environmental treatments were inverted relative to treatments from Phase 2 of Period 1, retrieved oocytes were graded (A, B, C, and D), and fertilized with frozen-thawed semen. Embryo development was evaluated on Days 3 and 7. An increased number of follicles was obtained 3 days after DFR in both HSC and CON groups (P < 0.05), but no significant difference was observed in follicle count between those two experimental groups. HS significantly reduced the total number of oocytes, Grade A and B oocytes, cleaved zygotes, and developed blastocysts (P < 0.05, respectively). No significant difference was observed in the blastocyst development rates. These results suggest that a follicular population can be increased in the HS-affected cow by employing DFR. However, the embryo yield per OPU can remain impaired.