Treatment of luteal phase mares with the aromatase inhibitor letrozole does not change characteristics of the estrous cycle, ovarian function, and embryo characteristics.

Letrozole is a potent and highly selective third-generation aromatase inhibitor. In the present study, we aimed to investigate the effects of letrozole treatment of non-pregnant mares on estrous cycle characteristics. Cyclic, non-lactating mares (n = 9) were included. After estrus synchronization, mares were monitored to detect ovulation (d0:) of the treatment cycle. Each three mares were randomly assigned to one of three treatments: control, 0.5, and 1 mg/kg body weight letrozole in three consecutive cycles (d7 to d13 of the treatment cycle). Mares were inseminated in the estrus after treatment and the day of ovulation was recorded (dOv). Embryo collection was performed 8 days after ovulation (dflush) of the post treatment cycle. Blood samples for analysis of progesterone, estradiol, testosterone and anti-Mullerian hormone were collected repeatedly. Antral follicle counts (AFC) were determined on d0, d4, d7 and d13 of the treatment and dOv and dflush of the post-treatment cycle. There were no effects of letrozole treatment on estrous cycle characteristics, pregnancy rate, and embryo characteristics. There were no differences in AFC among treatments, but follicle counts in the size classes 15 - <20 mm, 20 - <25 mm, 25 - <30 mm and ≥ 30 mm changed over time (p < 0.05). Letrozole treatment did not affect any hormone concentrations in serum. The results of this study scrutinize the suitability of this aromatase inhibitor for assessment of estradiol function in non-pregnant horses.