Predicting ovulation in the mare on the basis of follicular growth and serum oestrone sulphate and progesterone levels.

Follicular growth and hormone determinations were used to predict ovulation in the mare. Thirty Finnhorse mares were used for the investigation and 38 oestrous cycles were studied. The mares were examined by rectal palpation and ultrasonography every 6 hours during late oestrus. Daily blood samples were obtained for progesterone and oestrone sulphate determination. The preovulatory follicle grew 3 mm a day up to 2 days before ovulation. The size then remained constant, before diminishing by 2-3 mm during the last 12 hours. The maximal diameter of the follicle was 43 +/- 4 mm. In 89% of the follicles the round shape became more irregular before ovulation. During the last 24 hours before ovulation 37 of 38 follicles were regarded as mature on rectal palpation. The oestrone sulphate level was highest 24-48 hours before ovulation, the first decrease being observed most commonly around ovulation (+/- 1 day). The size of the follicle was the most reliable criterion in the prediction of ovulation.