COMPARISON OF CIRCULATING ESTRADIOL-17-BETA AND FOLLICULOGENESIS DURING THE BREEDING-SEASON, AUTUMN TRANSITION AND ANESTRUS IN THE MARE

One of the phenomena characteristic of the vernal transition into cycl icity in the mare is the variability between follicular size and conco mitant estradiol secretion. Researchers1 have suggested that early tra nsitional follicles of the spring may be estrogenically incompetent, g aining the ability to synthesize estrogens only immediately prior to t he first ovulation of the year. It is unknown if a similar phenomenon is operating during the autumnal transition due to limited study of th is period. Therefore, the association between follicular development a nd plasma estradiol concentrations in mares during their progress into anestrus was compared to the same relationships during the breeding s eason. Estrous behavior, plasma estradiol-17beta (E2) and progesterone (P4) concentrations, and follicular growth of 11 horse mares was eval uated from September 6 through December 31, 1989. For comparison, five summer cycling mares were monitored from July 10 through August 13, 1 991. Size of the largest follicle > 25 mm in diameter during estrus wa s compared by regression and correlation analysis with daily plasma E2 concentrations for summer and fall cycles. Smaller E2 concentrations/ mm follicular size were evident during the fall compared with summer. Correlations were higher (p < .05) for summer (r = .75) than for autum nal cycles (r = .47). The variability between mares in relation to the se two parameters was also higher (p<.0001) during the autumn transiti on. The follicular phase was lengthened in autumnal cycles, as well as an increase in mean follicular size. These results document a diminis hed estradiol production relative to follicular size with approaching a cyclicity. Estrogenic incompetence of autumnal follicles may be a fa ctor contributing to these phenomena.