The effect of period (bimester) within breeding season upon reproductive characteristics of mares was studied, using 147 oestrous cycles of 100 mares in two breeding seasons. They were under follicular control, and artificial insemination was used with extended transported/cooling semen at 14ºC for 3.5 hours. Mares were grouped according to ovulation bimester of each oestrous cycle, within the breeding season as follow: October/November, December/January and February/March. The results showed that conception rate at first cycle was higher on Dec/Jan (70%) than on the other two periods (42.9% on Oct/Nov, and 28.6% on Feb/Mar); conception rate per cycle was higher on Dec/Jan (63.5%), than on Feb/Mar (29.6%), and pregnancy efficiency was higher on Oct/Nov and Dec/Jan (4.4; 6.0) than on Feb/Mar (2.3). No differences between bimester were found on size of ovulation follicle and follicular growth time. It is concluded that elimination of the extreme breeding season months is feasible without loosing herd reproductive efficiency.
Equine; breeding season; fertility; ovulation
