title: Effects of gonadotropin-releasing hormone at initiation of the 5-d timed artificial insemination (AI) program and timing of induction of ovulation relative to AI on ovarian dynamics and fertility of dairy heifers.
abstract: Two experiments evaluated the effects of the first GnRH injection of the 5-d timed artificial insemination (AI) program on ovarian responses and pregnancy per AI (P/AI), and the effect of timing of the final GnRH to induce ovulation relative to AI on P/AI. In experiment 1, 605 Holstein heifers were synchronized for their second insemination and assigned randomly to receive GnRH on study d 0 (n = 298) or to remain as untreated controls (n = 307). Ovaries were scanned on study d 0 and 5. All heifers received a controlled internal drug-release (CIDR) insert containing progesterone on d 0, a single injection of PGF(2α) and removal of the CIDR on d 5, and GnRH concurrent with timed AI on d 8. Blood was analyzed for progesterone at AI. Pregnancy was diagnosed on d 32 and 60 after AI. Ovulation on study d 0 was greater for GnRH than control (35.4 vs. 10.6%). Presence of a new corpus luteum (CL) at PGF(2α) injection was greater for GnRH than for control (43.1 vs. 20.8%), although the proportion of heifers with a CL at PGF(2α) did not differ between treatments and averaged 87.1%. Progesterone on the day of AI was greater for GnRH than control (0.50 ± 0.07 vs. 0.28 ± 0.07 ng/mL). The proportion of heifers at AI with progesterone <0.5 ng/mL was less for GnRH than for control (73.8 vs. 88.2%). The proportion of heifers in estrus at AI did not differ between treatments and averaged 66.8%. Pregnancy per AI was not affected by treatment at d 32 or 60 (GnRH = 52.5 and 49.8% vs. control = 54.1 and 50.0%), and pregnancy loss averaged 6.0%. Responses to GnRH were not influenced by ovarian status on study d 0. In experiment 2, 1,295 heifers were synchronized for their first insemination and assigned randomly to receive a CIDR on d 0, PGF(2α) and removal of the CIDR on d 5, and either GnRH 56 h after PGF(2α) and AI 16h later (OVS56, n = 644) or GnRH concurrent with AI 72 h after PGF(2α) (COS72; n = 651). Estrus at AI was greater for COS72 than for OVS56 (61.4 vs. 47.5). Treatment did not affect P/AI on d 32 in heifers displaying signs of estrus at AI, but COS72 improved P/AI compared with OVS56 (55.0 vs. 47.6%) in those not in estrus at AI. Similarly, P/AI on d 60 did not differ between treatments for heifers displaying estrus, but CO S72 improved P/AI compared with OVS56 (53.0 vs. 44.7%) in those not in estrus at AI. Administration of GnRH on the first day of the 5-d timed AI program resulted in low ovulation rate and no improvement in P/AI when heifers received a single PGF(2α) injection 5 d later. Moreover, extending the proestrus by delaying the final GnRH from 56 to 72 h concurrent with AI benefited fertility of dairy heifers that did not display signs of estrus at insemination following the 5-d timed AI protocol.
authors.affiliation: Department of Animal Sciences, University of Florida, Gainesville 32611, USA.
authors.forename: F S
authors.initials: FS
authors.lastname: Lima
authors.forename: H
authors.initials: H
authors.lastname: Ayres
authors.forename: M G
authors.initials: MG
authors.lastname: Favoreto
authors.forename: R S
authors.initials: RS
authors.lastname: Bisinotto
authors.forename: L F
authors.initials: LF
authors.lastname: Greco
authors.forename: E S
authors.initials: ES
authors.lastname: Ribeiro
authors.forename: P S
authors.initials: PS
authors.lastname: Baruselli
authors.forename: C A
authors.initials: CA
authors.lastname: Risco
authors.forename: W W
authors.initials: WW
authors.lastname: Thatcher
authors.forename: J E P
authors.initials: JE
authors.lastname: Santos
mesh_terms: D000818:Animals; D002417:Cattle; D003612:Dairying; D004973:Estrus Synchronization / Q000187:drug effects; D005260:Female; D005298:Fertility / Q000187:drug effects*; D007987:Gonadotropin-Releasing Hormone / Q000008:administration & dosage / Q000494:pharmacology*; D006728:Hormones / Q000008:administration & dosage / Q000494:pharmacology; D007315:Insemination, Artificial / Q000662:veterinary*; D010053:Ovary / Q000187:drug effects*; D010060:Ovulation / Q000187:drug effects*; D010062:Ovulation Induction / Q000662:veterinary*; D011247:Pregnancy; D011897:Random Allocation; D013997:Time Factors
chemical_list: D006728:Hormones; D007987:Gonadotropin-Releasing Hormone