TIME OF ONSET AND OVARIAN STATE PRIOR TO ENTRY INTO WINTER ANESTRUS

The reproductive activity of 18 light horse mares was followed from Se ptember 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 follo wed by ultrasonography daily during estrus and once weekly during dies trus/anestrus. Progesterone (P4) was measured by RIA on plasma drawn d aily or weekly. Entry into anestrus was considered complete if a mare did not exhibit cyclic estrous behavior, had no follicles greater-than -or-equal-to 25 mm, and maintained P4 less-than-or-equal-to 1 ng/ml fo r at least 30 days. Two mares continued cycling while 16 mares ceased ovarian activity at diverse times during the study period. Winter cycl es were the same as summer cycles in length, but not in P4 secretion. Entry into anestrus did not occur until November and peak numbers occu rred in January. The time to first ovulation in the spring (March-Apri l) was not affected by the time of entry into anestrus. Spontaneously prolonged corpus luteum (SPCL) function (P4 > 1 ng/ml > 30 days) reach ed a peak level by December (28% of mares) and rapidly declined in Jan uary. Mares entered anestrus from three different ovarian states: afte r resolution of a SPCL (n = 3), or following luteolysis of a normal CL (n=4), or following follicular atresia (n = 9). These three methods o f entry into anestrus were not equally distributed throughout the autu mnal transitional period (p<0.001). Half of all anestrous mares entere d this state prior to the winter solstice, and did so following follic ular atresia. Those who entered anestrus later in the winter did so fo llowing 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) following luteolysis. These data indicate that true ane strus is achieved later than previously reported and that luteal funct ion is altered during winter.