The effect of hormone treatments (hCG and cloprostenol) and season on the incidence of hemorrhagic anovulatory follicles in the mare: a field study.

The association between use of hormone treatments to induce estrus and ovulation and the incidence of hemorrhagic anovulatory follicles (HAFs) was studied in a mixed population of mares (Equus caballus) during two breeding seasons in a commercial breeding clinic. Mares treated with cloprostenol (CLO) were more likely to develop HAFs than were mares with spontaneous cycles (P<0.001) or those treated with human chorionic gonadotropin alone (P=0.08). There was no significant effect of season on the incidence of HAFs. The mean (+/-SEM) interval from CLO treatment to beginning of HAF development was 6.1+/-0.5 d. Age of mares with HAF cycles was not different (12+/-1.3 yr; P>0.05) from that of mares with ovulatory cycles (10.5+/-1.5 yr).