Modifications to Ovsynch improve fertility during resynchronization: Evaluation of presynchronization with gonadotropin-releasing hormone 6 d before initiation of Ovsynch and addition of a second prostaglandin F2α treatment

Abstract

Lactating Holstein cows (n = 897) were randomly assigned to a 2 × 2 factorial arrangement of treatments to compare the main effects of presynchronization with GnRH 6 d before beginning an Ovsynch protocol (±GnRH) and a second PGF2αtreatment 24 h after the first (1 vs. 2 PGF2α) on pregnancies per artificial insemination (P/AI). This resulted in the following 4 treatments: (1) an Ovsynch protocol (GPG, control); (2) presynchronization with GnRH followed by an Ovsynch protocol (GGPG); (3) an Ovsynch protocol with a second PGF2αtreatment (GPPG); and (4) presynchronization with GnRH followed by an Ovsynch protocol with a second PGF2αtreatment (GGPPG). All cows were submitted for first timed artificial insemination (TAI) using a Presynch Ovsynch protocol, and cows detected in estrus after the second PGF2αtreatment of the Presynch portion of the protocol were inseminated and removed from the experiment. Nonpregnant cows were resynchronized using an Ovsynch protocol initiated 32 ± 3 d after artificial insemination. Blood samples were collected at the first GnRH treatment (G1), at the PGF2αtreatment (PGF), and at the last GnRH treatment (G2) of the Ovsynch protocol and were assayed for progesterone (P4) concentrations. Overall, P/AI tended to be greater for cows receiving a second PGF2αtreatment compared with cows not receiving the second PGF2αtreatment (40 and 37% for GGPPG and GPPG treatments, respectively, vs. 33 and 32% for GGPG and GPG treatments, respectively). Interestingly, treatment effects on P/AI were detected only for resynchronized cows receiving second and greater TAI, but not for cows receiving first TAI. Fewer cows presynchronized with GnRH had low (<0.5 ng/mL) P4 at G1 than cows not presynchronized (13 vs. 25%), and P4 was greater at PGF2αfor cows presynchronized with GnRH (4.4 vs. 4.0 ng/mL). Cows receiving 2 PGF2αtreatments had lower P4 at G2 than cows receiving 1 PGF2αtreatment (0.15 vs. 0.35 ng/ mL). Differences in P4 at PGF2αwere detected only for resynchronized cows and not for cows submitted for first TAI. We conclude that presynchronization with GnRH 6 d before beginning an Ovsynch protocol tended to increase P4 at the first GnRH treatment of an Ovsynch protocol, and that a second PGF2αtreatment 24 h after the first decreased P4 at TAI, thereby increasing P/AI in cows resynchronized for second and greater TAI.