Progesterone-based fixed-time artificial insemination protocols for dairy cows: GnRH vs. estradiol benzoate at initiation and estradiol cypionate vs. estradiol benzoate at the end.

• Grant number: 16/13869-3
• Support Opportunities: Regular Research Grants - Publications - Scientific article
• Start date: August 01, 2016
• End date: January 31, 2017
• Field of knowledge: Agronomical Sciences - Veterinary Medicine - Animal Reproduction
• Principal Investigator: Roberto Sartori Filho
• Grantee: Roberto Sartori Filho
• Host Institution: Escola Superior de Agricultura Luiz de Queiroz (ESALQ). Universidade de São Paulo (USP). Piracicaba , SP, Brazil

Abstract

The objectives were to evaluate ovarian dynamics and fertility comparing two treatments at the start of a progesterone (P4)-based fixed-time artificial insemination (FTAI) protocol, and two treatments at the end of the protocol. Thus, 1,035 lactating Holstein cows were assigned in a random phase of the estrous cycle to one of four treatments using a completely randomized design with a 2x2 factorial arrangement. At the beginning of the protocol (d-10), cows received GnRH or estradiol benzoate (EB) and at the end EB (d-1) or estradiol cypionate (ECP; d-2), resulting in four treatments: GnRH-EB, GnRH-ECP, EB-EB and EB-ECP. All cows received an intravaginal P4 device on d-10 which was removed on d-2. Cows also received PGF2± (PGF) on d-3 and d-2. The FTAI was performed on d0. Ovaries were evaluated by ultrasound for corpus luteum (CL) presence and regression (d-10 and d-3) and follicle measurements (d-10 and d0), as well as the uterus for percentage pregnant per AI (P/AI; d32 and d60). Blood samples were collected (d-10 and d-3) for P4 measurements. Treatment with GnRH rather than EB tended to increase P/AI on d32 (38.2 vs. 33.7%) and on d60 (32.9 vs. 28.9%). More cows treated with GnRH had CL on d-3 compared with EB-treated cows (77.3 vs. 58.3%), due to less CL regression between d-10 and d-3 (24.7 vs. 43.8%) and more cows with a new CL on d-3 (35.9 vs. 25.0%). Cows treated with GnRH also had greater P4 concentrations on d-3 than EB cows (3.4 vs. 2.0 ng/mL). Increased circulating P4 at the start of the protocol (d-10) decreased the probability of ovulation to EB or GnRH at that time. Cows from GnRH group also ovulated a larger diameter follicle at the end of the protocol (15.5 vs. 14.7 mm). No difference between EB and ECP in P/AI on d32 (34.8 vs. 37.0) and d60 (30.8 vs. 31.0%) or in pregnancy loss (11.1 vs. 15.4%) was detected and there was no interaction between treatments for P/AI. Independent of treatment, cows with CL on d-10 and d-3 had the greatest P/AI on d60 (36.9%). In conclusion, treatments at the end of the protocol were similar for ECP or EB and there was no additive effect or interactions on P/AI between treatments. However, cows treated with GnRH rather than EB on d-10 had less luteolysis and tended to have greater P/AI, probably because P4 concentrations were greater during the protocol. Finally, regardless of treatments, cows with CL at the beginning of the protocol as well as at the time of PGF had greater fertility. (AU)