Follicular Dynamics Following Cloprostenol Administration in the Diestrous Mare

Prostaglandin F2α (PGF) treatment is routinely used in the reproductive management of mares to induce luteolysis and a subsequent return to estrus. The objectives of this study were to 1) evaluate the interval from administration of a single, 250 µg intramuscular dose of cloprostenol sodium (Estrumate® Schering-Plough Animal Health Corp., Summit, NJ) to ovulation in the mare with considerations of follicle size, mare age, and season at time of administration; 2) evaluate the fate of large follicle (≥35 mm) at the time of PGF administration; and 3) determine incidence of hemorrhagic anovulatory follicle (HAF) formation following PGF administration in mid-diestrus to 275 American Quarter Horse mares. A total of 529 estrous cycles were evaluated. Continuous data were compared using a one way ANOVA (analysis of variance) with post hoc analysis by student’s t-test. Significance was set at p < 0.05. Categories were analyzed by Chi Square analysis. Data are presented as mean ± S.E.M. The average interval to ovulation following cloprostenol administration was 8.4 ± 2.5 days. The interval from PGF administration to subsequent spontaneous ovulation was inversely proportional to the diameter of the largest follicle at the time of treatment, if the follicle went on to ovulate (Table). In some mares, the largest diestrous follicle regressed and was replaced by another follicle on either the ipsilateral or contralateral ovary.  […]