Reproductive hormone profiles in mares during the autumn transition as determined by collection of jugular blood at 6 h intervals throughout ovulatory and anovulatory cycles

Abstract

The aim was to define precisely the FSH secretion pattern in mares during the two ovulatory cycles before, and for 24 days after, the last ovulation of the season and to compare this with the profiles of other reproductive hormones and follicular growth to identify changes which may lead to the termination of follicular cycles. Jugular blood was collected every 6 h from ten light horse mares for 6 weeks in autumn. Samples were assayed for FSH, LH, prolactin, inhibin, oestrone conjugates and progesterone. Luteolysis occurred earlier and periovulatory oestrone, but not inhibin, concentrations were significantly lower in the last than in the second to last cycles. In ovulatory and anovulatory cycles, daily mean FSH concentrations were low at the expected time of ovulation and high between days 9 and 11 (day 0 = ovulation), which were usually after luteolysis. However, the periovulatory FSH nadir was prolonged in the last compared with the second to last cycles, and the difference between peak and trough values was not significant in anovulatory cycles. Between day 5 and day 8, the FSH interpulse interval was approximately 2 days, and did not vary in successive cycles. The LH profile also showed progressive changes as mares entered acyclicity; the surge terminated sooner in the last than in the second to last cycles, and failed to occur when expected in acyclicity. Sporadic prolactin pulses occurred at luteolysis in a similar proportion of ovulatory and anovulatory cycles. These results indicate that inadequate gonadotrophin stimulation in early dioestrus may be a critical event leading to suboptimal follicular and luteal development, and eventually acyclicity. Moreover, the time relationships amongst changes in pituitary and ovarian hormones and follicular growth become increasingly disrupted during the autumn transition, which may contribute to the cessation of cyclicity.