Background and Aim: Ultrasonographic fetal sexing is of utmost economic importance for horse breeders. Relatively, a few studies have been conducted to determine fetal sex in mare using transrectal Doppler ultrasound. This study aimed to compare two sexing techniques, two-dimensional (2D) mode and color Doppler ultrasonography. Materials and Methods: The study was conducted on 39 mares under field conditions. Examinations were performed using the ultrasonic model device (Medison SonoAce Pico, South Korea), equipped with real-time 3-7 MHz convex multifrequency transducer. Fetal sex diagnosis was carried out in two periods of pregnancy, early period (57-80 days of gestation) and late period (80-150 days of gestation). Results: No difference (p=0.4) was observed between the efficiency of the 2D mode and Doppler ultrasound in sex determination with the respective frequency of 74% and 85%. The best time to sex the early fetus was between 57 and 70 days of gestation and between 90 and 120 days for the older fetus. Conclusion: The accuracy of sex determination can be increased using Doppler mode compared to 2D mode, especially in the early period between 57 and 70 days, in male more than female sex.

To advance the time of the first estrus with ovulation, we administered intravaginally a progesterone-releasing devices PRID^® Delta to 42 acyclic Arab-Barb mares for 11 days at different seasons: winter (n = 10), spring (n = 14), summer (n = 9) and autumn (n = 9). Twenty-seven of 39 mares that kept their coils responded to the PRID^® Delta treatment with estrus and ovulation during the 42-day observation period, in winter (4/10, 40%) mares, in spring (9/13, 69%), in summer (7/7, 100%) and autumn (7/9, 78%) mares (P <0,05). At Day 2 of PRID® Delta treatment, mean initial of progesterone concentrations was significantly increased in all mares in winter (7.20 ± 0.49 ng mL-1), spring (7.30 ± 0.64 ng mL-1), summer (7.5 ± 0.58 ng mL-1 ) and autumn ( 7.60 ± 0.71 ng mL-1) (P <0,05). Total concentration of progesterone (area under curve (AUC)) during the treatment period revealed highest values in spring (67.95 ± 2.40 ng mL-1h-1) followed by autumn (65.20 ± 1.37 ng mL-1 h-1), winter (54.19 ± 7.00 ng mL-1 h-1) and summer 52.23 ± 3.32 ng mL-1 h-1; P < 0.05). In conclusion, administration of the intravaginal device releasing progesterone PRID® Delta was able to induce estrus and ovulation in mares at different times of the year. However, the efficacy of the treatment was not satisfactory in terms of efficacy compared to the season (low response rate in winter) and synchrony of intervals from removal of PRID^® Delta to ovulation.