In a first experiment, 11 Finnhorse mares were examined every six hours during late oestrus by rectal palpation and ultrasonography to determine the time of ovulation. The mares were inseminated over one to three subsequent cycles, 6-12 (n=5), 12-18 (n=5), 18-24 (n=5) and 24-30 (n=5) hours after ovulation. Pregnancies were terminated by prostaglandin injection 21 days after insemination. All mares inseminated within 18 hours of ovulation conceived but no mare inseminated 24 hours or more after ovulation conceived.In a second experiment, 14 mares were examined every day at about the same time and inseminated after ovulation had been detected, within 0-27 hours of ovulation. Pregnancies were terminated by prostaglandin injection 25 days after insemination and the animals inseminated again. Forty-six per cent of the mares (13 out of 28) were pregnant 16 days after insemination but there was early embryonic death in five mares, i. e. only 29 % of the mares became and remained pregnant.The equine ovum seems to be capable of being fertilized for at least 18 hours after ovulation. However, the high incidence of early embryonic death in the second experiment suggests that the viability of the embryo may have decreased.

Summary 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. Zusammenfassung Vorhersage der Ovulation bei der Stute aufgrund des Follikelwachstums, der östronsulfat‐ sowie der Progesteronkonzentration des Serums Zur Vorhersage der Ovulation wurden bei der Stute das Follikelwachstum sowie Hormonbestimmungen herangezogen. Für die Untersuchungen wurden 30 Stuten verwendet. Insgesamt wurden 38 Brunstzyklen untersucht. Die Stuten wurden während der späten Phase des östrus alle 6 Stunden mittels rektaler Palpation und Ultraschall untersucht. Für die Progesteron‐ und östronsulfatbestim‐mungen wurde täglich Blutproben entnommen. Der präovulatorische Follikel wuchs bis 2 Tage vor der Ovulation 3 mm pro Tag. Die Größe blieb dann konstant, bevor sich der Follikel in den letzten 12 Stunden um 2–3 mm verkleinerte. Der maximale Follikeldurchmesser betrug 43 ± 4 mm. Bei 89% der Follikel wurde die runde Form vor der Ovulation unregelmäßig. Bei rektaler Palpation wurden während der letzten 24 Stunden vor der Ovulation 37 von 38 Follikeln als reif beurteilt. Der östronsulfatspiegel war 24–48 Stunden vor der Ovulation am höchsten; die erste Abnahme wurde gewöhnlich um die Ovulation herum (± 1 Tag) beobachtet. Die Follikelgröße war für die Vorhersage der Ovulation das verläßlichste Kriterium.

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