The oestrous cycle of the mare.

In eleven non-pregnant mares examined for 2 years, oestrous cycle length was 20-6 days (range 13 to 34) excluding any anoestrous periods. The duration of oestrus was 5-7 days (range 1 to 24); from February to May it was 7-6 days (range 2 to 24) and from May to November 4-8 days (range 1 to 10). The majority of ovulations occurred between 16.00 and 08.00 hours and 78% of the mares ovulated within 48 hr of the end of oestrus. Mean follicular diameter was 45 mm on the day of ovulation and there was a 25-5% incidence of multiple ovulations. It was uncommon for oestrus to occur without ovulation, whereas ovulation without oestrus occurred in 6/11 mares, including one mare which ovulated without oestrus on 32/34 occasions. The CL was palpable after ovulation for 8-9 days (range 1 to 18). Dioestrus averaged 15-4 days (range 6 to 25). Ovulations during dioestrus, unaccompanied by signs of oestrus, occurred in 10/11 mares but cycle length was not affected. A syndrome of spontaneous prolongation of the CL, accompanied by follicular activity, without any signs of oestrus for periods of 2 to 3 months was observed in six of the mares.