The Correlation of Endogenous Progesterone Concentration in Diestrus on Early Pregnancy Rate in Thoroughbred Mares.

This research study hypothesized and examined whether progesterone levels in Thoroughbred mares after ovulation were different between pregnant and non-pregnant mares. The findings showed higher progesterone levels in pregnant mares, but no definitive minimum required progesterone concentration could be pinpointed.

Study Objective and Methodology

• The objective of this research was to study the correlation between the concentration of endogenous progesterone five days after ovulation and early pregnancy rates in Thoroughbred mares. The researchers hypothesized that there would be no difference in progesterone levels between pregnant and non-pregnant mares.

• A prospective cohort study was conducted involving 275 mares from five different stud farms in the Waikato region of New Zealand. The study was performed during the 2018 breeding season and used 34 individual stallions.
• Blood samples were collected five days after ovulation to measure progesterone concentrations. Early pregnancy was confirmed after two weeks by transrectal palpation and ultrasonography of the mares’ reproductive tract.

Findings

• The research showed that the concentration of progesterone was higher in pregnant mares compared to those determined non-pregnant at 14 days after ovulation (6.4 ± 3.0 ng/ml vs. 5.5 ± 3.3 ng/ml respectively).
• Analysis revealed a negative association between the age of the mare and pregnancy rate, implying that the pregnancy rate decreases as the mare ages. However, the age of the mare showed no influence on the progesterone concentration at day 5 after ovulation.

Implications and Future Scope

• Despite the higher serum progesterone concentration observed in mares that were pregnant at day 14 post ovulation, the research did not identify a definitive minimum required progesterone concentration that could be linked definitively to pregnancy.
• The study suggests that further research is needed to investigate whether synthetic progestogens can replace natural progesterone in order to enhance the pregnancy rate in naturally bred mares.