Although it is well known that the ovulation occurs during a period of time after LH surge in dogs, there are few reports of observing the entire process of development, ovulation and luteinization of each follicle. This study aimed to detect the ovulation kinetics by ultrasonography in combination with progesterone monitoring and therefore identify the time-range of the ovulation process in a dog. Daily transabdominal ultrasonography and progesterone monitoring were conducted for 24 natural oestrus cycles of Labrador Retrievers. Ovarian follicles were observed as anechoic structure with contours before ovulation. Ovulation (follicular collapse) was defined as when follicles became cloudy and contours obscure by transabdominal ultrasonography. Ultrasound imaging was capable of identifying the day of ovulation for 94.7% (178/188) of the follicles through the appearance of collapsed follicle or corpus luteum. Ovulation was observed between LH 0 (the day of LH surge) and LH 5, with 48.0%, 33.5% and 15.0% for LH 2, LH 3 and LH 1, respectively. The total number of ovulations on LH 2 and LH 3 accounted for 81.5% (141/173) of the total ovulation in 24 cycles examined. Ovulation occurred in 12 cycles for 2 d and for 3 d in 12 cycles. Seventeen cycles (70.8%) with multiple days of ovulation showed the largest number of ovulations on LH 2. The average follicle diameter 3 d before the LH surge was less than 5 mm, then exceeded 5 mm 2 d before the LH surge. The average follicle diameter at the time of ovulation (follicular collapse) was 6.1 ± 1.0 mm (n = 118). On the day before ovulation, the average diameters of the follicles ovulated on LH 1, LH 2 and LH 3 were 5.0 ± 0.7 mm, 5.8 ± 1.2 mm and 6.2 ± 1.3 mm, respectively. There was a significant difference in the follicle diameter between LH1 and LH2 (p < .001), LH2 and LH3 (p < .05), and LH1 and LH3 (p < .001). Suggesting that it is difficult to estimate the ovulation day based on follicle size. This study showed that combination of ultrasonography with progesterone monitoring could follow follicular development, ovulation and luteinization of the ovary in Labrador Retrievers. The direct visualization of the ovulation was achieved in a non-invasive, labour-friendly way. Furthermore, the time-range of the ovulation process was clarified in a dog. These results may contribute to an accurate understanding of the optimum timing of mating and improved breeding efficiency, including artificial insemination and embryo transfer for Labrador Retrievers.
Keywords: dog; follicle; ovulation; progesterone; ultrasonography.
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