Selected ovarian ultrasonographic characteristics during vernal transition are useful to estimate time of first ovulation of the year.
It is important to get mares pregnant as early as possible after vernal transition and thus, identification signs of impending 1st ovulation of the year are warranted. To identify clinical indicators of an approaching first ovulation of the year, mares were teased with a stallion for oestrous detection starting January 3 and subjected to ultrasonographic examination. Day of first appearance of uterus oedema, follicular wall invagination, intrafollicular echogenicity, double contour of the follicle wall, increase in granulosa thickness, follicular wall hyperechogenicity and appearance of pear-shaped follicles was registered, as well as follicle diameter and number. Seventy per cent of the mares had anovulatory oestrous periods of 4.6 ± 3.6 days, with an interoestroual interval of 12.5 ± 12.2 days. Number of anovulatory oestruses per mare was 2.4 ± 2.3. Uterine oedema occurred in 77% of the mares, 32.4 ± 25.6 days before ovulation. Invagination of the follicular wall appeared in 44.4% of the animals, 24.5 ± 18.4 days before ovulation. Intrafollicular echogenicity was seen in all mares and double contour of the follicle was seen in 77% of the animals. Both last two characteristics appeared 1-72 days before ovulation. Increased thickness of the granulosa occurred in 66% of the mares, 1-19 days before ovulation. Pear-shaped follicles and follicular wall hyperechogenicity were detected 3 or less days before the first ovulation, in 44.4% and 55.5% of mares, respectively. Mean number of follicles > 15 mm decreased at least 16 days before ovulation. We concluded that no isolated characteristic was a reliable indicator. However, increase in granulosa thickness, formation of a pear-shaped follicle and follicular wall hyperechogenicity, associated with the reduction of the number of follicles > 15 mm in diameter to < 3, resulted in the first ovulation of the year in 44-67% of the transitional mares, 1-19 days after the characteristics appeared.
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ICBAS -Abel Salazar Biomedical Institute and CECA - Centre for the Study of Animal Sciences, University of Porto, Vairão, Portugal.
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D000818:Animals; D004971:Estrus / Q000502:physiology; D005260:Female; D006736:Horses / Q000502:physiology*; D008297:Male; D010053:Ovary / Q000000981:diagnostic imaging* / Q000502:physiology*; D010060:Ovulation / Q000502:physiology*; D012621:Seasons; D014463:Ultrasonography