hCG-induced ovulation in thoroughbred mares does not affect corpus luteum development and function during early pregnancy.

This research paper investigates the impact of human chorionic gonadotropin (hCG)-induced ovulation on the development and function of the corpus luteum, and progesterone levels in thoroughbred mares during early pregnancy. The research shows that hCG-induced ovulation does not affect these parameters.

Study Design and Methodology

• The study was conducted on lactating mares with two groups: one (C=12) was allowed to ovulate spontaneously whereas the other (hCG=21) was induced to ovulate using hCG. The induced group received 2500 IU hCG intravenously at the first day of behavioural oestrus when the dominant follicle size was more than 35mm but less than or equal to 42mm and were mated 12-24 hours afterwards.
• The control group mares in oestrus were mated when the dominant follicle size was equal to or more than 45mm.
• The dominant follicle before ovulation, corpus luteum, and gestational sac were measured using ultrasound and progesterone levels were measured using a radioimmunoassay (RIA).
• Blood samples and ultrasound exams of the corpus luteum were done on specific days after ovulation and the gestational sac was measured from day 12 after ovulation in pregnant mares. Non-pregnant mares were followed until oestrus returned.

Data Analysis and Results

• The data were analysed considering four subgroups: hCG-P (hCG treated and pregnant), hCG-NP (hCG treated and not pregnant), C-P (control and pregnant), and C-NP (control and not pregnant).
• Preovulatory follicular size was significantly smaller in the hCG-treated mares compared to the control group.
• All hCG-treated mares ovulated within 24-48 hours after treatment and presented similar oestrus duration as the control group.
• Corpus luteum size in pregnant mares showed the same development pattern from day 4 through day 35, with some minor differences during initial establishment. The corpus luteum size in hCG-P was smaller than in C-P on days 1 and 3, whereas, in hCG-NP the corpus luteum size was greater than in C-NP on day 3.
• The size of the corpus luteum remained stable until day 90 in hCG-P mares but displayed a transient, and apparently non-functional increase on days 40 and 45 in C-P mares.
• No differences were observed in the progesterone level patterns between pregnant or non-pregnant subgroups, respectively.

Conclusion

• The study concluded that hCG-induced ovulation does not affect corpus luteum development, nor does it change the progesterone levels in early pregnancy.
• One cycle pregnancy rate seemed to be lower in hCG treated mares, however, the overall season pregnancy rates were similar to the control group.