title: Synchronization of ovulation in cattle with an aromatase inhibitor-based protocol.
abstract: A study was designed to determine the effect of stage of the estrous cycle on the proportion of animals that ovulated and the synchrony of ovulation of heifers treated with an aromatase inhibitor-based protocol. Forty-eight heifers were treated intramuscularly with 500 μg of cloprostenol (PGF) followed by 100 μg of GnRH 24 hours later to serve as control data for comparison of the ovulatory response to a subsequent aromatase inhibitor protocol. Daily ultrasound examinations were done to determine the incidence of and interval to ovulation. At the time of ovulation (Day 0), heifers were assigned randomly to five day-groups (n = 8-11/group) and given an intravaginal device containing 3 g of letrozole for 4 days starting on Day 0, 4, 8, 12, or 16. At the time of device removal, heifers were given PGF followed by GnRH 24 hours later. Ultrasound examinations were done daily from 2 days before device insertion to 9 days after the posttreatment ovulation. The preovulatory follicle diameter after letrozole treatment was larger in the Day 4 group compared to the Day 0 and 16 groups and intermediate in the Day 8 and 12 groups (P < 0.001). Compared to control data, the percentage of heifers that ovulated after letrozole treatment was greater (87.1% vs. 69.4%, respectively; P < 0.05) as was the synchrony of ovulation (residuals: 0.24 ± 0.07 vs. 0.68 ± 0.13; P < 0.01). The day on which letrozole treatment was initiated did not affect the proportion of heifers that ovulated or the interval to ovulation. Plasma estradiol concentrations at the time of removal of the letrozole device in the Day 0 and 4 groups was lower (P < 0.05) than in the corresponding controls. Estradiol concentrations in the Day 8 and 12 groups did not differ from already low concentrations in the respective controls. Corpus luteum diameter profiles and progesterone production were not affected by day-group although reduced luteal lifespan after letrozole treatment was observed and requires further investigation. In summary, a protocol involving a letrozole-impregnated intravaginal device for 4 days, PGF treatment at device removal, and GnRH 24 later resulted in a greater ovulation rate and greater synchrony of ovulation than in heifers not given letrozole. Results suggest that the protocol may be initiated effectively at random stages of the estrous cycle and may provide impetus for further studies to assess the efficacy of a letrozole-based synchronization protocol for fixed-time insemination.
authors.affiliation: Department of Veterinary Biomedical Sciences, Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, Saskatchewan, Canada.
authors.forename: M J
authors.initials: MJ
authors.lastname: Yapura
authors.affiliation: Department of Large Animal Clinical Sciences, Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, Saskatchewan, Canada.
authors.forename: R J
authors.initials: RJ
authors.lastname: Mapletoft
authors.affiliation: Department of Obstetrics, Gynecology, and Reproductive Sciences, College of Medicine, University of Saskatchewan, Saskatoon, Saskatchewan, Canada.
authors.forename: R A
authors.initials: RA
authors.lastname: Pierson
authors.affiliation: Department of Veterinary Biomedical Sciences, Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, Saskatchewan, Canada.
authors.forename: J
authors.initials: J
authors.lastname: Singh
authors.affiliation: Department of Veterinary Biomedical Sciences, Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, Saskatchewan, Canada. Electronic address: gregg.adams@usask.ca.
authors.forename: G P
authors.initials: GP
authors.lastname: Adams
mesh_terms: D000282:Administration, Intravaginal; D000818:Animals; D047072:Aromatase Inhibitors / Q000494:pharmacology*; D002417:Cattle / Q000502:physiology*; D003008:Cloprostenol / Q000494:pharmacology; D003338:Corpus Luteum / Q000000981:diagnostic imaging; D004958:Estradiol / Q000097:blood; D004973:Estrus Synchronization / Q000379:methods*; D005260:Female; D007987:Gonadotropin-Releasing Hormone / Q000494:pharmacology; D000077289:Letrozole; D009570:Nitriles / Q000008:administration & dosage / Q000494:pharmacology*; D010060:Ovulation / Q000187:drug effects; D010061:Ovulation Detection / Q000379:methods / Q000662:veterinary; D011374:Progesterone / Q000097:blood; D011897:Random Allocation; D014230:Triazoles / Q000008:administration & dosage / Q000494:pharmacology*
chemical_list: D047072:Aromatase Inhibitors; D009570:Nitriles; D014230:Triazoles; D007987:Gonadotropin-Releasing Hormone; D003008:Cloprostenol; D011374:Progesterone; D004958:Estradiol; D000077289:Letrozole
keywords: Aromatase inhibitor; Cattle; Estradiol; Letrozole; Ovulation; Synchronization