hCG-Induced Ovulation in Thoroughbred Mares Does Not Affect Corpus luteum Development and Function During Early Pregnancy

Abstract

Our aim was to compare Corpus luteum (CL) development and blood plasma concentration of progesterone ([P(4)]) in thoroughbred mares after spontaneous (Control: C) or human chorionic gonadotrophin (hCG)-induced ovulation. Lactating mares (C = 12; hCG = 21) were daily teased and mated during second oestrus post-partum. Treated mares received 2500 IU hCG i.v. at first day of behavioural oestrus when dominant follicular size was > 35, < 42 mm and mated 12-24 h after. Control mares in oestrus were mated with dominant follicular size >= 45 mm. Dominant follicle before ovulation, CL and gestational sac were measured by ultrasound and [P(4)] by radioimmunoassay (RIA). Blood sampling and ultrasound CL exams were done at days 1, 2, 3, 4, 8, 12, 16, 20, 25, 30, 35, 40, 45, 60 and 90 after ovulation and gestational sac from day 12 after ovulation in pregnant (P) mares; non-pregnant (NP) were followed until oestrus returned. Data analyses considered four subgroups: hCG-P, hCG-NP, C-P and C-NP. Preovulatory follicular size was smaller in hCG mares than in C: 39.2 +/- 2.7 mm vs 51.0 +/- 1.8 mm (p < 0.0001). All hCG mares ovulated 24-48 h after treatment and presented similar oestrus duration as controls. C. luteum size in P mares showed the same pattern of development through days 4-35, presenting erratic differences during initial establishment. Thus, on days 1 and 3, CL was smaller in hCG-P (p < 0.05); while in hCG-NP, CL size was greater than in C-NP on day three (p = 0.03). Corpus luteum size remained stable until day 90 in hCG-P mares, while in C-P a transient and apparently not functional increase was detected on days 40 and 45 (p < 0.05) and the decrease from day 60 onwards, made this difference to disappear. No differences were observed in [P(4)] pattern between P, or between NP subgroups, respectively. So, hCG-induced ovulation does not affect CL development, neither [P(4)] during early pregnancy. One cycle pregnancy rate tended to be lower in hCG mares while season pregnancy rates were similar to controls.