Day of prostaglandin F(2α) administration after natural ovulation affects the interval to ovulation, the type of ovulated follicle, and the failure to induce ovulation in cows

The factors that affect the interval to ovulation, the type of ovulated dominant follicle (DF), and the cause of anovulation after prostaglandin (PG) treatment were investigated. Nine cows were assigned to six groups (54 cows in total) but the group size was later fixed at eight cows (48 in total). They received 25 mg tromethamine dinoprost as dinoprost on Day 6 (Group D6), Day 7 (Group D7), Day 8 (Group D8), Day 9 (Group D9), Day 10 (Group D10), or Day 11 (Group D11) after natural ovulation (Day 0). If the DF did not ovulate, then the cow was assigned to Group NO. In Group D6, the 1st DF ovulated in all cows 4 days after PG treatment, whereas in Groups D9, D10, and D11, the 2nd DF ovulated in all cows 4 to 7 days after PG treatment. In 10 cows, the DF did not ovulate, and late anovulation was significantly higher in Group D6 cows than in Group D11 cows. The progesterone (P(4)) levels decreased to less than 1 ng/ml in all groups on the day after PG treatment. The estradiol-17β (E(2)) levels began to increase after PG treatment and peaked at 2 days before ovulation in the cows that ovulated. In anovulated cows, E(2) tended to be higher and there was no clear E(2) peak in some cows. These results indicated that the number of days to ovulation, the type of ovulated DF, and anovulation were affected by factors that were associated with the DF when it was producing E(2).