Characteristics of cyclicity in maiden thoroughbred mares in the United Kingdom.

This study investigates the regular reproductive cycles of 12 un-bred, thoroughbred mares in the UK over 58 cycles. The research examines the behaviors and mechanisms of oestrus (heat) and ovulation (egg release), including the synchronicity of these processes, the size development of pre-ovulatory follicles, the percentage of cycles where multiple follicles ovulated, and the prevalence of uterine swelling just prior to ovulation.

Basics of Cyclicity in Thoroughbred Mares

• The study included twelve maiden (nofault) thoroughbred mares, grouped in two, wherein their cycles were monitored over a total of 58 cycles. These cycles consisted of oestrus or heat, which is when the mares are receptive to mating, and ovulation, the release of an egg from an ovary.
• On average, the oestrus phase lasted for about 5.3 days and in 60% of the cycles, the ovulation took place during the last two days of oestrus. This finding suggests a fairly predictable oestrus-ovulation timing amongst the observed mares, which may be crucial for breeders in timing insemination or natural mating.

Follicle Maturation and Ovulation

• The researchers also measured the diameter of single-ovulating preovulatory follicles (small sacs in the ovary that mature into eggs) the day before the ovulation occurred. They found that on average, these follicles measured 41.5 mm on the day before ovulation.
• Moreover, these follicles grew by an average of 2.5 mm per day during the seven days leading up to ovulation. This observation provides a valuable piece of evidence regarding the follicle development process in thoroughbred mares.

Multiple Ovulations and Uterine Edema

• Significantly, the study also discovered that more than one follicle ovulated in about 33% of the cycles. This included seven instances of double ovulations and 12 instances of dioestrous ovulations (occurring in the non-heat phase of the cycle).
• All of the oestrus mares, and 25% of the mares with dioestrous ovulations, had uterine oedema, a swelling of the uterus typically indicating oestrus, on the day prior to ovulation. This potentially highlights a correlation between the onset of oestrus, ovulation, and observable physical changes such as uterine swelling.

Overall, the study provides valuable insights into the cyclical reproductive behaviors of maiden thoroughbred mares, which can be instrumental for breeders and veterinarians in managing horse breeding and health.