Uncoupling of the equine reproductive axes during transition into anoestrus

The reproductive activity of light horse mares (n=12) was monitored each day from 3 September until 29 January, or until the mares entered anoestrus, by behaviour evaluation, ultrasonography and blood sampling. Follicles, corpora lutea and ovulation, as well as oestradiol, progesterone, and LH and FSH concentrations, were analysed to determine a reproductive profile for the transition into anoestrus. The results of the present study indicate that light horse mares progress through four phases during the autumn transition into anoestrus: (i) normal cycles; (ii) aberrant cycles; (iii) anovulation, with significant follicular activity; and (iv) anoestrus. One of the first changes observed was a progressive decrease in mean progesterone concentrations during normal cycles (summer: 7.1 +/- 0.4 ng ml(-1); last cycle: 3.6 +/- 0.2). Regression analysis indicates that large follicles progressively lose their ability to produce oestradiol in autumn (third last cycle: 8.5 +/- 1.3 pg ml(-1); last cycle: 5.7 +/- 0.5). During the last ovulatory surge, LH concentrations decreased approximately 50% in 6 of 8 mares. The duration of the follicular phase increased with each cycle approaching anovulation. However, the diameters of the follicles ovulated did not differ. The duration of the luteal phase during ovulatory cycles did not change. Four of the 12 mares developed spontaneously prolonged corpora lutea and were eliminated from the analysis. An anovulatory follicular growth phase occurred immediately before anoestrus in 7 of 8 mares. FSH preceded follicular growth during all cycles and persisted throughout the anovulatory period. During anoestrus, plasma oestradiol, progesterone and LH concentrations remained at basal concentrations. FSH concentrations remained high in 3 of 8 mares but did not cause follicular growth. It is concluded that ovarian and pituitary events become uncoupled during the transition into anoestrus.