Retinol and cholecalciferol affect buserelin-induced estrous in anestrous mares

In winter anestrous, prolonged melatonin secretion inhibits gonadotropin-releasing hormone (GnRH). However, synthetic GnRH analogues such as buserelin can stimulate follicular development. We aimed to investigate clinical relationship between retinol, cholecalciferol, and buserelin for inducing estrous in anestrous mares (Equus caballus). We used a total of twenty-one anestrous mares, randomly divided into three groups of seven animals. Group A received retinol, cholecalciferol, and buserelin; group B received buserelin; and group C was control. Groups A, B, and C had 71.42% (n = 5; 95% CI: 26.30%-96.56%), 28.57% (n = 2; 95% CI: 16.56%-73.70%), and 0% mares in estrous, respectively. A significantly greater number of group A mares exhibited estrous (P = 0.005) and higher ovarian follicular size (P = 0.001) compared to group C. Serum estrogen was significantly higher in group A compared to Group B (P = 0.03) and C mares (P = 0.001). In multiple correspondence analysis (MCA) factor map, treatment with retinol, cholecalciferol, and buserelin was clustered with estrous mares having serum estrogen levels > 40 pg/ml and follicular size > 30 mm. The variance explained by the first two dimensions of MCA was 87.83%. Supplementing with retinol and cholecalciferol improved the rate of buserelin-induced estrous in anestrous mares. Further research is necessary to determine underlying mechanisms.