Plasma Estradiol and Endometrial Edema Profile in Acyclic Mares After Single Administration of 17‐β Estradiol, Estradiol Benzoate and Estradiol Cypionate

A longer endometrial exposure to estradiol before progesterone has been shown to be beneficial in cyclic and acyclic recipient mares. Therefore, the selection of an estradiol ester that promotes longer endometrial exposure to estradiol using a single administration would be advantageous when preparing acyclic mares as embryo recipients. This study investigated plasma estradiol profiles in acyclic mares after a single administration of 17-β estradiol (17-β), estradiol benzoate (EB) and estradiol cypionate (EC), and the correlation between plasma concentrations and endometrial edema. Fifteen non-cyclic mares were divided into groups 17-β (n = 5), EB (n = 5) or EC (n = 5), receiving a single dose of 10 mg of the respective hormone. Blood sample collections and transrectal ultrasonography were performed every 6 h from hour 0 to 12, every 12 h from 12 to 48 h, and every 24 h from 48 to 120 h after hormone administration. Five of the acyclic mares were used during the breeding season as a cyclic control. Greater median concentrations were detected using EB (38.6 pg/mL; p < 0.05). For 17-β, peak concentration was observed at 6 h (29.7 pg/mL) and decreased 24 h after administration (5.9 pg/mL; p < 0.05). In the EC group, there was a modest peak starting from 12 h (11.7 pg/mL; p < 0.05), remaining relatively constant until 120 h. A more rapid increase of edema to moderate and high scores was found when using 17β estradiol, although edema scores and persistence until Day 5 were similar among the oestrogens used. A correlation between estradiol concentration and endometrial edema was only seen when using EC, and this hormone also produced the most similar concentration values to those found in natural cycling mares. Therefore, it is likely that EC would be a suitable hormone for preparing acyclic mares as embryo recipients.