Luteolytic effects of cloprostenol sodium in lactating dairy cows treated with G6G/Ovsynch

The probability of a pregnancy decreases substantially in lactating dairy cows treated with Ovsynch if luteolysis is delayed or incomplete. Two PGF2α products are currently approved in the United States for luteolysis in lactating dairy cattle, dinoprost tromethamine and cloprostenol sodium. Cloprostenol has a longer half-life compared with dinoprost, is more resistant to endogenous metabolism, and is maintained in circulation longer. We hypothesized that cloprostenol could reduce the time to complete luteolysis compared with dinoprost because of differences in half-life. Lactating dairy cows received the same presynchronization strategy (G6G; 25 mg of PGF2α - 2 d - 100 μg of GnRH - 6 d - 100 μg of GnRH - 7 d - final PGF2α treatment). At the time of the final PGF2α, cows (n = 35) were randomly assigned to receive either 500 μg of cloprostenol or 25 mg of dinoprost. Blood samples were collected daily before and serially after PGF2α treatment to analyze circulating concentrations of progesterone (P4) and estradiol (E2). Ultrasound examinations of ovaries were performed to measure sizes of follicles and corpora lutea (CL) and determine time of ovulation. Considering only cows with complete luteolysis, mean circulating P4 was lower for cows given cloprostenol than for those given dinoprost between 0 and 12 h postinjection, but not at 24, 36, or 48 h. A rapid decrease in P4 was observed 1 h after PGF2α (6.54 ± 0.27 to 3.77 ± 0.22 ng/mL) followed by a complete rebound 1 h later (3.77 ± 0.22 to 5.07 ± 0.31 ng/mL) followed by a steady decline in both treatment groups. Serum concentrations of E2 were greater at 48 h posttreatment in cloprostenol-treated cows (2.74 ± 0.15 pg/mL) than in dinoprost-treated cows (2.37 ± 0.19 pg/mL). Cows that did not have complete luteolysis did not ovulate (0/7) during the 6-d period following treatment. Time to complete luteolysis and ovulation was 29.1 ± 1.1 versus 29.4 ± 1.7 and 101 versus 103 h posttreatment in cloprostenol compared with dinoprost. A negative relationship was observed between P4 at 12 h posttreatment and concentrations of E2 48 h posttreatment (b = −0.6905; R2 = 0.23). In summary, cows treated with cloprostenol had lower concentrations of P4 for the first 12 h following treatment and subsequently greater concentrations of E2 compared with dinoprost, although no differences were observed in these 2 PGF2α analogs for time to complete luteolysis or time to ovulation.