Animal Reproduction Science, 23 (1990) 169-180 169 Elsevier Science Publishers B.V., Amsterdam Effect of day of prostaglandin F2~ treatment on selection and development of the ovulatory follicle in heifers J.P. Kastelic, L. Knopfand O.J. Ginther Department of Veterinary Science, University of Wisconsin-Madison, Madison, WI 53 706, U.S.A. (Accepted 4 May 1990) ABSTRACT Kastelic, J.P., Knopf, L. and Ginther, O.J., 1990. Effect of day of prostaglandin F2,~ treatment on selection and development of the ovulatory follicle in heifers. Anita. Reprod. Sci., 23:169-180. In previous studies of heifers with two follicular waves during an estrous cycle, the dominant follicle of Wave 1 was first detected ultrasonically on approximately the day of ovulation (Day 0) when its diameter was 4-5 ram. On average, it grew linearly for 6 days (growing phase), remained the same size for 6 days (static phase), and then regressed (regressing phase). The dominant follicle of Wave 2 was first detected on approximately Day 9 and became the ovulatory follicle. In the present experi- ment, nonbred and bred heifers were treated with a luteolytic dose of prostaglandin F2,~ (25 mg) on Days 5, 8, or 12, when the dominant follicle of Wave 1 was expected to be in the growing, static, and regressing phase, respectively. There were no significant effects of breeding status on any end point. The hypothesis that growth of the dominant follicle during Wave 1 and response to prostaglandin F2,~ treatment is different between bred and nonbred heifers was not supported. Ovulation occurred from the dominant follicle of Wave 1 in 5 of 5, 6 of 6 and 0 of 4 heifers treated on Days 5, 8, and 12, respectively (P<0.005). Wave 2 was not detected in the Day-8 heifers, but was the origin of the ovulatory follicle in the Day-12 heifers. The results supported the hypothesis that the dominant folli- cle of Wave 1 is viable (capable of ovulation ) before detection of Wave 2. For heifers treated on Days 5, 8, and 12, the ovulatory follicle had a mean diameter of 13.8, 17.3, and 11.8 mm, respectively, on the day of treatment and a mean diameter of 16.0, 19.5, and 16.4 ram, respectively, on the day prior to ovulation (significant increase between treatment and day prior to ovulation for each group). The results supported the hypothesis that the static-phase dominant follicle of Wave 1 is capable of further growth after luteolysis, even though it has apparently reached maximum diameter. The interval from treatment to ovulation was significantly shorter in Day-5 heifers (mean, 3.0 days) than in Day-12 heifers (mean, 4.5 days). In summary, the viable dominant follicle present at the time of luteolysis increased in diameter and became the ovulatory follicle. INTRODUCTION A wave of ovarian follicular growth in cattle involves the synchronous de- velopment of a group of follicles with each wave culminating in the develop- ment of a dominant follicle. In studies utilizing daily ultrasonic monitoring 0378-4320/90/$03.50 © 1990 -- Elsevier Science Publishers B.V.