Preovulatory Follicle Dynamics, and Ovulatory and Endometrial Responses to Different Doses of hCG and Prediction of Ovulation in Mares

Abstract

A more precise characterization of the effects of human chorionic gonadotropin (hCG) treatment on the preovulatory follicle is needed to improve reproductive efficiency in broodmare management. The objectives of this study were (1) to study the effects of different hCG doses within and among three consecutive estrous cycles on ovulatory response, preovulatory follicle dynamics, and endometrial echotexture; (2) to study the temporal relationship among changes in preovulatory follicle diameter, endometrial echotexture, and estradiol levels; and (3) to analyze a system for prediction of ovulation. In experiment 1, mares were treated with 2,500 IU hCG, 1,500 IU hCG, 500 IU hCG, or saline (controls) and examined by ultrasonography every 6 hours across three consecutive preovulatory periods (n = 135). Similar treatment groups were utilized in experiment 2 (n = 177 preovulatory periods) with examinations performed every six, 12, or 24 hours to test the effectiveness of a prediction of ovulation system. Effective hCG doses (2,500 and 1,500 IU) led to cessation of growth rate and reduction of the preovulatory follicle diameter concurrently with a reduction in systemic estradiol concentration and endometrial echotexture score. A system of prediction of ovulation, based on preovulatory follicle ultrasonographic signs of impending ovulation, was highly accurate for the diagnosis of positive and negative outcomes in both hCG-treated and control groups. However, the effectiveness of this prediction system was reduced due to the false-negative diagnoses that were a reflection of a lack of sensitivity for the detection of nonovulation.