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J. Dairy Sci. 90:1281-1288
© American Dairy Science Association, 2007.

Ovarian Traits After Gonadotropin-Releasing Hormone-Induced Ovulation and Subsequent Delay of Induced Luteolysis in an Ovsynch Protocol1

J. S. Stevenson2, M. A. Portaluppi and D. E. Tenhouse

Department of Animal Sciences and Industry, Kansas State University, Manhattan 66506-0201

2 Corresponding author: jss{at}k-state.edu

Our objective was to determine whether delaying the PGF2{alpha} injection by 24 or 48 h after the first GnRH injection in an Ovsynch protocol (from a standard 7 d) altered ovarian characteristics in lactating dairy cows. Beginning 9 d after removal of a progesterone-releasing controlled internal drug release (CIDR) insert and injection of PGF2{alpha} (d 6.4 of the estrous cycle), 36 Holsteins (average body weight = 707 ± 12 kg and body condition score = 2.3 ± 0.1) were administered 100 µg of GnRH (81 ± 2 d in milk) and assigned randomly to receive a treatment injection of PGF2{alpha} 7, 8, or 9 d later. Timed artificial insemination was performed at 48 h after PGF2{alpha} at which time a second injection of GnRH was administered. Ovarian structures were mapped by ultrasonography on d 0 (first GnRH injection); on d 2 to determine responses to the first GnRH injection; at PGF2{alpha} injection; and daily thereafter through 72 h after PGF2{alpha} to monitor ovulation of preovulatory follicles. Blood was collected on d 0, 2, at PGF2{alpha} injection, and at 24 and 48 h after PGF2{alpha} to monitor serum changes in estradiol-17ß (E2-17ß) and progesterone (P4). Based on serum P4 and ovarian exams, 2 cows were eliminated because of anestrus and their failure to ovulate a follicle in response to the first GnRH injection. Two other cows in which luteolysis failed to occur after PGF2{alpha} treatment also were eliminated. Final numbers of cows per treatment were: 7 d (n = 13), 8 d (n = 9), and 9 d (n = 10). Twenty-nine of 32 cows ovulated (90.6%) in response to the first GnRH injection. Of those cows not ovulating in response to the first GnRH injection, 2 had 1 original corpus luteum and 1 had 2 original corpora lutea. Despite a 24- or 48-h delay between first GnRH and PGF2{alpha} injections, the diameter (mm) and volume (mm3) of the ovulatory follicle did not differ among treatments: 14.3 ± 0.6 and 1,526 ± 62 at 7 d; 14.1 ± 0.8 and 1,479 ± 97 at 8 d; and 15.3 ± 0.9 and 1,490 ± 69 at 9 d. In all 32 cows, at least 1 follicle ovulated after treatment, but ovulation rates did not differ: 1.2 ± 0.1, 1.1 ± 0.1, and 1.3 ± 0.2, respectively, for the 7-, 8-, and 9-d treatments. Serum concentrations of E2-17ß did not differ among treatments. Four cows in the 7-d treatment were inseminated 24 h late and were excluded before assessing conception rates, which were 5/9 (55.6%), 5/9 (55.6%), and 1/10 (10%), respectively. We conclude that delaying PGF2{alpha} injection by 24 h had no effect on outcomes.

Key Words: Ovsynch • ovulation • follicle • timing of luteolysis






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