CHARACTERISTICS OF CYCLICITY IN MAIDEN THOROUGHBRED MARES IN THE UNITED-KINGDOM

The characteristics of the cyclicity of 12 maiden thoroughbred mares k ept in two groups were studied over a total of 58 cycles. On average, oestrus lasted 5.3 days and in 60 per cent of the cycles ovulation occ urred in the last two days of oestrus. Oestrus and ovulation tended to be synchronised in each group of mares. The mean diameter of single-o vulating preovulatory follicles on the day before ovulation was 41.5 m m and during the seven days before ovulation they grew 2.5 mm/day. Mor e than one follicle ovulated in 19 (33 per cent) of the cycles (seven double ovulations and 12 dioestrous ovulations). All the oestrous mare s and 25 per cent of the mares with dioestrous ovulations had uterine oedema on the day before ovulation.