Effectiveness of Selected Methods of Estrus Cycle Manipulation in Mares During the Breeding Season: A Retrospective Clinical Study

Hormones used for the induction of estrus and ovulation are widely applied in veterinary practice. The objectives of this study were: (1) to assess the effect of cloprostenol on estrus induction, (2) to evaluate the effect of hCG on changes in the endometrial edema pattern and its direct impact on the timing of ovulation in relation to the progression of the breeding season, and (3) to assess the effects of cloprostenol and hCG on the induction of multiple ovulations (MO) and multiple pregnancies (MP).The first group of mares (n = 77), with spontaneous estrus, received a dose of hCG to induce ovulation, while the second group (n = 40) had estrus induced by cloprostenol, followed by an hCG injection. The pregnancy rate was 55.8% in the first group and 65% in the second group (p < 0.073). The frequency of MO was higher in the second group (27.5%) compared to 3.9% in the first group (p < 0.0006). However, this treatment did not affect the frequency of MP. Endometrial edema scores at the time of hCG administration significantly decreased over time in both groups. Cloprostenol can increase the frequency of MO without affecting MP.