title: Effect of initial GnRH and duration of progesterone insert treatment on the fertility of lactating dairy cows.
abstract: The study compared response to prostaglandin F2α (PG), synchrony of ovulation and pregnancy per AI (P/AI) in a 5- vs a 7-day Ovsynch + PRID protocol and investigated whether the initial GnRH affects P/AI in lactating dairy cows. Two hundred and seventy-six cows (500 inseminations) were assigned to one of four timed-AI (TAI) protocols: (i) PRID-7G; 100 μg GnRH im, and a progesterone-releasing intravaginal device (PRID) for 7 days. At PRID removal, PG (500 μg of cloprostenol) was given im. Cows received the second GnRH treatment at 60 h after PRID removal and TAI 12 h later. (ii) PRID-5G; as PRID-7G except the duration of PRID, treatment was 5 days and PG was given twice (12 h apart). (iii) PRID-7NoG; as PRID-7G except the initial GnRH, treatment was omitted. (iv) PRID-5NoG; as PRID-7NoG except the duration of PRID, treatment was 5 days. Response to treatments and pregnancy status at 32 and 60 days after TAI was determined by ultrasonography. The percentage of cows ovulating before TAI was greatest in PRID-7G (17.1%), and the percentage of cows that did not have luteal regression was greatest in PRID-5G (9.5%). The overall P/AI at 32 and 60 days did not differ among TAI protocols. However, during resynchronization, cows subjected to the 5-day protocols had greater (p < 0.05) P/AI (45.3% vs 33.6%) than cows subjected to the 7-day protocols. Pregnancy loss between 32 and 60 days tended (p = 0.10) to be greater in cows that did not receive initial GnRH (14.8%) compared to those that received GnRH (8.2%). In conclusion, the PRID-5G protocol resulted in fewer cows responding to PG, but P/AI did not differ among TAI protocols. A 5-day protocol resulted in more P/AI in resynchronized cows, and cows that did not receive initial GnRH tended to experience more pregnancy losses.
authors.affiliation: Alberta Agriculture and Rural Development, Edmonton, AB, Canada.
authors.forename: M G
authors.initials: MG
authors.lastname: Colazo
authors.forename: D J
authors.initials: DJ
authors.lastname: Ambrose
mesh_terms: D000818:Animals; D002417:Cattle; D015237:Dinoprost / Q000494:pharmacology; D004334:Drug Administration Schedule; D004973:Estrus Synchronization / Q000379:methods; D005260:Female; D005298:Fertility / Q000187:drug effects*; D007987:Gonadotropin-Releasing Hormone / Q000494:pharmacology*; D007774:Lactation / Q000502:physiology*; D011247:Pregnancy; D011374:Progesterone / Q000008:administration & dosage / Q000494:pharmacology*
chemical_list: D007987:Gonadotropin-Releasing Hormone; D011374:Progesterone; D015237:Dinoprost