Uncoupling of the equine reproductive axes during transition into anoestrus.

The study analyzes the reproductive cycle of light horse mares as they enter a non-reproductive or anoestrus period. The researchers monitored mares’ behavior, ultrasonography and blood sampling to determine a reproductive profile during this transition, noting a decrease in progesterone concentrations and other hormonal changes.

Objectives and Methodology

• The research aims to understand the transition of horses into anoestrus, a period of sexual inactivity. The subjects of the study were 12 light horse mares.
• From 3rd September to 29th January, these mares had their reproductive activity examined, through behavioural observation, ultrasonography, and blood tests.
• The scholars scrutinized follicles, corpora lutea and ovulation, and concentrations of oestradiol, progesterone, and hormones LH and FSH to develop a reproductive profile for this transition period.

Findings of the Study

• The researchers found that horse mares pass through four stages during the autumn transition to anoestrus: normal cycles, aberrant cycles, anovulation involving significant follicular activity, and finally, anoestrus.
• One initial observation was a steady decrease in mean progesterone levels during typical cycles from summer at 7.1 ng ml(-1) to the final cycle at 3.6 ng ml(-1).
• Follicles lost their ability to manufacture oestradiol progressively in the autumn, as shown through regression analysis.
• The duration of the follicular phase increased with each cycle, heading towards anovulation, but the diameters of the follicles ovulated did not show any changes.
• The duration of the luteal phase showed no changes during ovulatory cycles, while four of the 12 mares developed spontaneously prolonged corpora lutea and were left out of the analysis.
• An anovulatory follicular growth phase ensued just before anoestrus among seven of the eight evaluated mares.
• FSH preceded follicular growth during all cycles and persisted during the anovulatory period. During anoestrus, plasma oestradiol, progesterone and LH concentrations stayed at basal levels.
• FSH concentrations remained high among three of the eight mares but did not induce follicular growth. As such, the study concludes that ovarian and pituitary events became uncoupled during the transition into anoestrus.