title: Fixed-time artificial insemination in replacement beef heifers after estrus synchronization with human chorionic gonadotropin or gonadotropin-releasing hormone.
abstract: We determined the effects of hCG on ovarian response, concentration of progesterone, and fertility in a fixed-time AI (TAI) protocol. Four hundred forty-four crossbred beef heifers were synchronized with the CO-Synch + CIDR (controlled internal drug-releasing insert) protocol. In addition, heifers were randomly assigned to 1 of 4 treatments in a 2 × 2 factorial arrangement of treatments with main factors being 1) pretreatment, no treatment (control), or treatment with 1,000 IU of hCG 14 d before the initiation of the CO-Synch + CIDR protocol and 2) treatment, administration of 1,000 IU of hCG or 100 µg of GnRH at CIDR insertion of the CO-Synch + CIDR protocol. Blood samples were collected from all heifers on d -21, -14, -7, 0, and 2 relative to PGF(2α) injection. Transrectal ultrasonography was used to examine ovaries in a subset of heifers (n = 362) on d -7 and 0 relative to PGF(2α), and to determine pregnancy status of all heifers on d 33 and 82 relative to AI. Pregnancy rates were similar for heifers pretreated with control (33.0%) or hCG (36.4%), whereas pregnancy rates were greater (P < 0.01) for heifers treated with GnRH (40.1%) compared with hCG (29.0%) at CIDR insertion. Heifers pretreated with hCG had more (P < 0.01) corpora lutea present on the day of CIDR insertion and the day of CIDR removal compared with untreated heifers. A greater proportion (P < 0.01) of heifers ovulated as a result of administration of hCG at the time of CIDR insertion (59.0%) compared with GnRH (38.7%). Heifers treated with hCG at CIDR insertion had greater (P < 0.01) concentrations of progesterone compared with those receiving GnRH at the time of CIDR removal (2.42 ± 0.13 vs. 1.74 ± 0.13 ng/mL; P < 0.01) and at fixed-time AI (0.52 ± 0.03 vs. 0.39 ± 0.03 ng/mL; P < 0.01). Therefore, hCG was more effective than GnRH in its ability to ovulate follicles and to increase concentrations of progesterone in beef heifers. Presynchronization with hCG 14 d before CIDR insertion did not alter pregnancy rates, whereas replacing GnRH with hCG at CIDR insertion decreased pregnancy rates.
authors.affiliation: Department of Animal Sciences, North Dakota State University, Fargo, ND 58108, USA.
authors.forename: C R
authors.initials: CR
authors.lastname: Dahlen
authors.forename: G H L
authors.initials: GH
authors.lastname: Marquezini
authors.forename: J E
authors.initials: JE
authors.lastname: Larson
authors.forename: G C
authors.initials: GC
authors.lastname: Lamb
mesh_terms: D000818:Animals; D002417:Cattle; D006063:Chorionic Gonadotropin / Q000494:pharmacology*; D004973:Estrus Synchronization / Q000187:drug effects*; D005260:Female; D005298:Fertility / Q000187:drug effects; D007987:Gonadotropin-Releasing Hormone / Q000494:pharmacology*; D006801:Humans; D007315:Insemination, Artificial / Q000379:methods / Q000662:veterinary*; D010053:Ovary / Q000000981:diagnostic imaging / Q000187:drug effects; D010060:Ovulation / Q000187:drug effects; D011374:Progesterone / Q000097:blood; D014463:Ultrasonography
chemical_list: D006063:Chorionic Gonadotropin; D007987:Gonadotropin-Releasing Hormone; D011374:Progesterone