Seasonal influence on ovarian activity: Winter anestrus and transition to cyclic activity

Abstract

It is conventionally assumed that mares con t inue ovu l a to ry cyc les throughout summer and early autumn, entering a transitional phase before ceasing cyclic activity in late autumn or early winter. From this time until spring, the ovaries remain in varying degrees of quiescence. Stimulated by increasing day length, ovaries show increased activity from March onwards until ovulatory cycles begin in April or early May. Because mares are not normally bred in the last few months of the year, little information is available on ovarian activity during this period. With the breeding season not starting until early February, few mares are examined routinely before this time. Many studies have investigated the effect of increasing daylight or of exposure to artificial lights but in most cases these regimes have not started until the beginning of the new year. To investigate the effect of time of year on the level of ovarian activity, two populations of mares were studied.