Time of onset and ovarian state prior to entry into winter anestrus

Abstract

The reproductive activity of 18 light horse mares was followed from September to May, 1989 and 1991. Five mares were evaluated from July to August, 1991. All mares were teased three times per week. Ovarian size, follicular growth, ovulation and corpus luteum development was followed by ultrasonography daily during estrus and once weekly during diestrus/anestrus. Progesterone (P4) was measured by RIA on plasma drawn daily or weekly. Entry into anestrus was considered complete if a mare did not exhibit cyclic estrous behavior, had no follicles > 25 mm, and maintained P4 < 1 ng/ml for at least 30 days. Two mares continued cycling while 16 mares ceased ovarian activity at diverse times during the study period. Winter cycles were the same as summer cycles in length, but not inP4 secretion. Entry into anestrus did not occur until November and peak numbers occurred in January. The time to first ovulation in the spring (March-April) was not affected by the time of entry into anestrus. Spontaneously prolonged corpus luteum (SPCL) function (P4 < 1 ng/ml < 30 days) reached a peak level by December (28% of mares) and rapidly declined in January. Mares entered anestrus from three different ovarian states: after resolution of a SPCL (n = 3), or following luteolysis of a normal CL (n-4), or following follicular atresia (n = 9). These three methods of entry into anestrus were not equally distributed throughout the autumnal transitional period (p<0.001). Half of all anestrous mares entered this state prior to the winter solstice, and did so following follicular atresia. Those who entered anestrus later in the winter did so following lysis of a CL or SPCL. Only one mare who attained anestrus late in the transition period managed follicular growth (ending in atresia and anestrus) followingluteolysis. These data indicate that true anestrus is achieved later than previously reported and that luteal function is altered during winter.