title: Effects of progesterone presynchronization and eCG on pregnancy rates to GnRH-based, timed-AI in beef cattle.
abstract: Three experiments were conducted to determine the effects of low-dose progesterone presynchronization and eCG on pregnancy rates to GnRH-based, timed-AI (TAI) in beef cattle (GnRH on Day 0, PGF(2alpha) on Day 7, with GnRH and TAI on Day 9, 54-56 h after PGF(2alpha)). Experiments 1 and 2 were 2 x 2 factorials with presynchronization (with or without a once-used CIDR; Days -15 to 0 in Experiment 1 and Days -7 to 0, with PGF(2alpha) at insertion, in Experiment 2), and with or without 400 IU eCG on Day 7 in suckled cows. In Experiment 3, suckled cows and nulliparous heifers were either presynchronized with a twice-used CIDR (Days -5 to 0) and PGF(2alpha) at insertion, or no treatment prior to insertion of a new CIDR (Days 0-7). Presynchronization increased (P<0.05) ovulation rate to GnRH on Day 0 (75.0% vs 48.7%, 76.7% vs 55.0%, and 60.0% vs 36.1% for Experiments 1, 2, and 3, respectively), increased the diameter of the preovulatory follicle in Experiments 1 and 2, and increased the response to PGF(2alpha) (regardless of parity) in Experiment 1 (P<0.01), and in primiparous cows in Experiment 2 (P<0.01). Effects of presynchronization on pregnancy rates (53.4% vs 54.1%, 57.7% vs 45.3%, and 54.3% vs 44.4% for Experiments 1, 2, and 3, respectively) were influenced by parity and eCG (P<0.05). Treatment with eCG had no effect (P>0.05) on the diameter of the preovulatory follicle (Experiment 1), or the response to PGF(2alpha) (Experiments 1 and 2), but tended (P=0.08) to improve pregnancy rates, especially in primiparous cows that were not presynchronized (P<0.01). However, the effects of eCG and presynchronization were not additive.
authors.affiliation: Agriculture and Agri-Food Canada, Brandon, Manitoba, Canada R7A 5Y3. jsmall@agr.gc.ca
authors.forename: J A
authors.initials: JA
authors.lastname: Small
authors.forename: M G
authors.initials: MG
authors.lastname: Colazo
authors.forename: J P
authors.initials: JP
authors.lastname: Kastelic
authors.forename: R J
authors.initials: RJ
authors.lastname: Mapletoft
mesh_terms: D000818:Animals; D002417:Cattle; D006063:Chorionic Gonadotropin / Q000008:administration & dosage / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004334:Drug Administration Schedule; D004973:Estrus Synchronization / Q000379:methods; D005260:Female; D007987:Gonadotropin-Releasing Hormone / Q000008:administration & dosage / Q000494:pharmacology*; D007315:Insemination, Artificial / Q000662:veterinary*; D011247:Pregnancy; D018873:Pregnancy Rate; D011374:Progesterone / Q000008:administration & dosage / Q000494:pharmacology*
chemical_list: D006063:Chorionic Gonadotropin; D007987:Gonadotropin-Releasing Hormone; D011374:Progesterone