Urinary oestrogen and plasma progesterone levels in non-pregnant mares.

This research observed eight non-pregnant mares over 14 months to monitor ovarian activity and hormone levels, finding that cyclical ovarian activity took place for 60% of the study duration, persistent luteal activity for 20% and no activity for the rest of the time. Various properties and patterns were detected, such as high estrogen levels during the follicular phase, correlation between luteolysis to ovulation and estrone levels, and occurance of persistent luteal activity more frequently during summer.

Study Design

• The study included eight non-pregnant mares that were observed over a 14-month period. The observation was carried out to record signs of estrus and ovarian activity.
• During this period, levels of urinary estrogens and plasma progesterone were also measured – key hormones involved in the reproductive cycle of mares.

Ovarian Activity

• Within the timeframe of the study, cyclical ovarian activity was noted 60% of the time. This means the mares’ ovaries were active and likely producing eggs and hormones in a regular cycle.
• Persistent luteal activity was observed in 20% of the timeframe. The luteal phase is part of the estrous cycle where the corpus luteum (a temporary endocrine structure in female ovaries) is present. Persistent luteal activity may indicate a continued production of progesterone.
• For the remaining period, no ovarian activity was recorded.

Hormonal Levels and Associations

• Patterns in hormonal level changes were observed. During the follicular phase which is a stage in the sexual cycle where follicles in the ovary mature, urinary estrogen levels increased, reaching a maximum value 2 days before ovulation.
• There was a noted correlation between the duration of interval from complete luteolysis (the degradation of the corpus luteum) to ovulation and peak values of estrone (a type of estrogen).
• High estrogen levels were recorded during the period of persistent luteal activity. Interestingly, this phase occurred more often during the summer months.

Seasonal Effects

• The research also highlighted some impactful seasonal effects. Ovarian inactivity was more common during winter, which was characterized by very low levels of progesterone and reduced estrogen levels (less than 50 ng/ml urine).
• Some individual differences between the mares were also noted, but the study does not elaborate in detail what these were.