title: Altering the time of the second gonadotropin-releasing hormone injection and artificial insemination (AI) during Ovsynch affects pregnancies per AI in lactating dairy cows.
abstract: Based on previous research, we hypothesized that Cosynch at 72 h [GnRH-7 d-PGF(2alpha)-72 h-GnRH + artificial insemination (AI)] would result in a greater number of pregnancies per AI (P/AI) than Cosynch at 48 h. Further, we hypothesized that P/AI would be improved to a greater extent when GnRH was administered at 56 h after PGF(2alpha) before AI at 72 h due to a more optimal interval between the LH surge and AI. Nine hundred twenty-seven lactating dairy cows (n = 1,507 AI) were blocked by pen, and pens rotated through treatments. All cows received GnRH followed 7 d later by PGF(2alpha) and then received one of the following: 1) GnRH + timed AI 48 h after PGF(2alpha) (Cosynch-48); 2) GnRH 56 h after PGF(2alpha) + timed AI 72 h after PGF(2alpha) (Ovsynch-56); or 3) GnRH + timed AI 72 h after PGF(2alpha) (Cosynch-72). Pregnancy diagnoses were performed by ultrasound at 31 to 33 d post-AI and again at 52 to 54 d post-AI. Overall P/AI were similar for the Cosynch-48 (29.2%) and Cosynch-72 (25.4%) groups. The Ovsynch-56 group had a greater P/AI (38.6%) than Cosynch-48 or Cosynch-72. Presynchronized first-service animals had greater P/AI than cows at later services in Cosynch-48 (36.2 vs. 23.0%) and Ovsynch-56 (44.8 vs. 32.7%) but not in Cosynch-72 (24.6 vs. 26.2%). Similarly, primiparous cows had greater P/AI than multiparous cows in Cosynch-48 (34.1 vs. 22.9%) and Ovsynch-56 (41.3 vs. 32.6%), but not Cosynch-72 (29.8 vs. 25.3%). In conclusion, we found no advantage to Cosynch at 72 h vs. 48 h. In contrast, we found a clear advantage to treating with GnRH at 56 h, 16 h before a 72-h AI, probably because of more-optimal timing of AI before ovulation.
authors.affiliation: Department of Dairy Science, University of Wisconsin, Madison 53706, USA.
authors.forename: D J
authors.initials: DJ
authors.lastname: Brusveen
authors.forename: A P
authors.initials: AP
authors.lastname: Cunha
authors.forename: C D
authors.initials: CD
authors.lastname: Silva
authors.forename: P M
authors.initials: PM
authors.lastname: Cunha
authors.forename: R A
authors.initials: RA
authors.lastname: Sterry
authors.forename: E P B
authors.initials: EP
authors.lastname: Silva
authors.forename: J N
authors.initials: JN
authors.lastname: Guenther
authors.forename: M C
authors.initials: MC
authors.lastname: Wiltbank
mesh_terms: D000818:Animals; D002417:Cattle / Q000502:physiology*; D015237:Dinoprost / Q000008:administration & dosage; D004973:Estrus Synchronization; D005260:Female; D007987:Gonadotropin-Releasing Hormone / Q000008:administration & dosage*; D007315:Insemination, Artificial / Q000379:methods / Q000662:veterinary*; D007774:Lactation; D016015:Logistic Models; D010060:Ovulation; D011247:Pregnancy; D013997:Time Factors
chemical_list: D007987:Gonadotropin-Releasing Hormone; D015237:Dinoprost