Ovarian activity of the mare during winter and spring at a latitude of 19° 21′ north

The ovulatory activity of mares located at 19° 21′N was monitored from November to June. Twenty-one mares were used (11 quarter-mile mares and 10 hybrids). Every week the mares were subjected to ultrasonographic observations of the ovaries, and blood samples for progesterone determinations were collected. Estrus activity was evaluated by teasing with a stallion. A mare was considered to be in an anovulatory state if progesterone concentrations during 5 or more consecutive samples were below 1 ng/mL. Cyclic ovulatory activity during the entire duration of the study was recorded in 62% of the mares. Only 38% of the mares (5 quarter-mile and 3 hybrid mares) had anovulatory periods, which lasted 128.5 ± 14.4 days (mean ± SEM) and occurred at some point between November and May. The duration of the anovulatory period varied from 70 to 196 days. Estrous behavior was shown during the anovulatory period in 75% (6/8) of the mares that had such a period. Most mares maintain regular ovulatory activity during the winter and spring at 19° 21′ north, and that a wide variation occurs in the duration of the anovulatory period of those mares that stop cycling at this latitude.