Effectiveness of selected methods for manipulating the oestrus cycle and ovulation in mares during the breeding season: A retrospective clinical study

Hormones used for the induction of oestrus and ovulation are widely applied in veterinary practice. The objectives of this study were to assess: (1) the effectiveness of cloprostenol in inducing oestrus, and (2) the influence of cloprostenol and human chorionic gonadotropin (hCG) on the incidence of multiple ovulations (MO) and multiple pregnancies (MP). The first group of mares (n = 77), which entered spontaneous oestrus, received hCG to induce ovulation, while the second group (n = 40) had oestrus induced with cloprostenol, followed by 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 significantly higher in the second group (27.5%) compared to the first group (3.9%; P < 0.0006). However, this treatment did not affect the incidence of MP. Endometrial oedema scores at the time of hCG administration decreased significantly over time in both groups. These results suggest that hormonal manipulation of the oestrous cycle during the breeding season and ovulation induction can help ensure satisfactory pregnancy rates. Moreover, cloprostenol can increase the frequency of MO without affecting MP rates.