Backgrounds: Progesteronic environment is essential for estradiol-based protocols and it is commonly provided using progestogen implants and intravaginal progesterone releasing devices. Yet limited information is available on application of injectable progesterone for this purpose.
Objectives: Therefore, the present study was conducted to understand the effect of two doses of progesterone (250 mg and 500 mg) at the initiation of an estradiol-based protocol on fertility of Holstein dairy cows.
Methods: All cows were presynchronized with Ovsynch protocol (GnRH, 7 days later PGF2α, and 3 days later GnRH), and 7 days later, were allocated to an estradiol-based breeding protocol [estradiol benzoate (EB), 7 days later PGF2α, 80 hours later GnRH, and 16 hours later artificial insemination (AI)]. Cows received only EB, EB plus 250 mg progesterone and EB plus 500 mg progesterone in EB (control; n = 89), EB+P250 (n = 81) and EB+P500 (n = 72) groups, respectively. Moreover, cows were subjected to transrectal ultrasonographic examination of the ovaries at EB administration to determine size of the largest follicle as well as presence of CL at the commencement of the estradiol-based breeding protocol. Pregnancy diagnosis and confirmation were implemented on days 30 and 60 after AI.
Results: There was a tendency for greater pregnancy per AI (P/AI) on day 30 post-insemination in EB+P500 than EB group and in cows with CL than cows without CL (0.05 ≤ P < 0.10). In addition, there was a tendency for greater P/AI on day 60 post-insemination in cows with CL than cows without CL (0.05 ≤ P < 0.10). Furthermore, pregnancy loss between days 30 and 60 post-insemination tended to be greater in EB+P500 than EB group (0.05 ≤ P < 0.10).
Conclusion: In conclusion, the present study showed that a single injection of 500 mg progesterone could enhance the effectiveness of an estradiol-based protocol regardless of CL presence at the beginning of breeding protocol. Yet this dose of progesterone escalated pregnancy loss, which could be a concern for this method.