Effects of progesterone on reproductive function in mares after parturition.

This research investigates the impact of progesterone hormone injections on the reproductive functions of mares post-parturition, revealing that varying doses of the hormone can delay or block estrus and ovulation in a significant number of experimental subjects, with implications on estrogen levels and possibilities of conception.

Effects of Progesterone Injections

• The research involved administering 100 mg of progesterone per day to mares, post birth (from days 5 to 14). The results showed that oestrus and ovulation were either blocked or delayed in 6 out of 9 mares. The rest managed to ovulate during treatment without showing symptoms of oestrus, representative of a category of horses exhibiting differing hormonal responses.
• The urinary estrogen concentrations in these mares appeared the highest between days 9 and 15 post-foaling. However, these peak levels depicted around half the amount regularly observed in untreated mares during the postnatal oestrus and ovulation. This implies the inhibitory effect progesterone hormone injections might have on estrogen concentrations in mares.

Different Doses of Progesterone

• An escalated dose of 200 mg progesterone per day demonstrated a similar effect: blocking oestrus and ovulation in two mares, with no further suppression in estrogen levels. This exhibits a potential saturation effect of progesterone on ovarian hormonal activity.
• Follicles (small sacs within the ovaries that contain immature eggs) that were interrupted from developing beyond a specific stage lost their ability to ovulate after withdrawing progesterone treatments, providing insights on the hormonal modulatory role progesterone plays on follicular growth and maturation processes.

No Significant Tissue Differences

• Endometrial biopsies, tissue samples taken from the mares’ uterus linings, of both progesterone-treated and control mares did not display any significant histological (microscopic tissue structure) differences on days 5, 10, and 15 post-foaling. This suggests the hormone-induced physiological changes do not have observable impacts on tissue morphology.

Progesterone Influence on Conception

• Injecting 100 mg of progesterone between days 1 and 10 following foaling resulted in delayed oestrus and ovulation in all six mares. This suggests that progesterone can possibly influence the reproductive cycle starting window in mares, allowing for flexible reproductive management.
• Of the 12 progesterone-treated mares, conception occurred in 8, as a result of mating during oestrous periods. This suggests that hormonal treatment in mares can lead to successful conception, despite the delayed estrous and ovulation after foaling.