178 The effects of heat stress on the number of ovarian follicles in the dairy cow

Ovum pickup (OPU) combined with IVF has facilitated bovine embryo production over the past few decades. However, heat stress (HS) can considerably compromise cattle herd productivity. Follicle ablation (FA) is known to induce a new follicular wave in the ovary by removing inhibiting factors of the present antral follicles. The objective of this study was to evaluate the effects of HS on the number of ovarian follicles at FA and OPU. Ten multiparous lactating Holstein cows (number of lactations: 2.8 ± 0.29, days in milk: 228.6 ± 21.55, mean ± SE) were randomly assigned to either Group 1 or Group 2 (n = 5) in a split-plot crossover design with two periods: 9 days in phase 1 and 10 days in phase 2 per period. In phase 1 (Period 1), all cows were kept in a tie-stall barn (20.9 ± 2.6°C) and fed ad libitum. On Day 9 of phase 1, GnRH (Gonadorelin hydrochloride, 200 µg) was given to all the cows. In phase 2 (Period 1), Group 1 was exposed to HS challenge (HSC) using an electric heat blanket (Thermotex Therapy Systems Ltd.) that generates temperature of 36°C. Group 2 was not challenged but was pair-fed to HSC, based on the decrease in the dry matter intake of HSC in the last 24 h (control: CON). On Day 7 of phase 2, all the follicles present in the ovaries were counted and classified into three groups based on their size assessed by transrectal ultrasonography: small, <3 mm; medium, 3–6 mm; and large, >6 mm. FA was done to all the large follicles with an ultrasound-guided transvaginal approach. Three days after FA (Day 10 of phase 2), all follicles present in the ovaries were counted, classified, and aspirated (OPU) with an ultrasound-guided transvaginal approach. After a 14-day washout period, the study was repeated with GnRH administration, FA, and OPU performed on the same days as in Period 1, but with HSC and CON in phase 2 inverted (Period 2). The difference in the number of follicles between HSC and CON was statistically analyzed using the MIXED procedure of SAS. A significant difference (P = 0.03) was observed between HSC and CON in the numbers of total follicles at OPU (Table 1). However, no difference was observed in those at FA or by size at OPU. In conclusion, HS did not affect the number of follicles 7 days after GnRH administration measured at FA. However, it could decrease the number of follicles 3 days after FA, suggesting that HS can compromise the recruitment of ovarian follicles early in a new follicular wave.