The oestrous cycle of the mare.

The research observed the oestrous cycle in non-pregnant mares over a two-year period, identifying the average length of the cycle and other related aspects such as ovulation timing and the occurrence of unusual phenomena like ovulation without oestrus.

Observation of Oestrous Cycle in Mares

• The research focused on examining the oestrous cycle of eleven mares over a period of two years. The creatures under study were not pregnant, and the sample does not include any periods when the animals were anoestrous.
• The non-pregnant mares had an average oestrous cycle length of 20.6 days, with observed cycles ranging from 13 to 34 days. This provides an understanding of the typical reproductive cycle in these mares.

Oestrus Duration

• The duration of oestrus (the period of sexual receptivity in the cycle) was also studied, showing an average length of 5.7 days. The range of duration, however, was quite wide, from just one day up to 24 days.
• The oestrus duration varied at different times of the year: from February to May, it was 7.6 days on average (ranging from 2 to 24 days), and from May to November, it shortened to an average of 4.8 days (ranging from 1 to 10 days).

Ovulation Timing and Multiple Ovulations

• The study found that most ovulations happened between 16:00 and 08:00 hours, and surprisingly, 78% of the mares ovulated within 48 hours of the end of oestrus.
• Multiple ovulations were observed in 25.5% of the instances, with the mean follicular diameter being around 45mm on the day of ovulation.

Ovulation and Oestrus Distinctions

• An interesting observation was that oestrus without ovulation was rare, while ovulation without oestrus was relatively common and occurred in 6 out of the 11 mares studied. This included one mare which ovulated without exhibiting any signs of oestrus on 32 out of 34 occasions.
• After ovulation, the Corpus Luteum (CL) was palpable for an average of 8.9 days, with a range observed between 1 and 18 days.

Dioestrus Period and Prolonged CL Syndrome

• The dioestrus period (the phase following ovulation when the mare is not sexually receptive) averaged 15.4 days, with a range of 6 to 25 days.
• Unusual instances of ovulations happening during the dioestrus period, unaccompanied by signs of oestrus, were detected in 10 out of 11 mares. Despite this unexpected occurrence, it did not affect the overall length of the oestrous cycle.
• In six mares, a syndrome of spontaneous prolongation of the Corpus Luteum was observed, characterized by follicular activity without any signs of oestrus, spanning periods of 2 to 3 months.