ASAS/ADSA Physiology: Reproductive Physiology

Experiment 1 evaluated pregnancy rates (PRs) when estradiol cypionate (ECP) replaced the second GnRH injection of a timed artificial insemination (TAI) protocol for lactating dairy cows. Cows were pre-synchronized with two injections of PGF2α (Lutalyse r , Pharmacia Corp., 25 mg; i.m.) given to multiparous cows (n=170) at 40 ± 3 and 54 ± 3 d postpartum (dpp) and to primiparous cows (n=201) at 62 ± 3 and 76 ± 3 dpp. The TAI protocol consisted of an injection of GnRH (Cystorelin r ; Merial Ltd., 100 μg, i.m.) at 68 ± 3 and 90 ± 3 dpp for multiparous and primiparous cows, respectively, followed by PGF2α 7 d later. Cows were injected with GnRH (Treatment I: Ovsynch, n=179) 48 h after PGF2α and inseminated 16 h later. In Treatment 2 (ECPsynch; n=192), ECP r (Pharmacia Corp., 1 mg, i.m.) was injected 24 h after PGF2α, and cows were inseminated 48 h later. Pregnancy was determined at 46 ± 3 d after TAI. For primiparous and multiparous cows, PRs for Ovsynch were 43.5 ± 6.9 and 30.6 ± 7.2 % compared to 50.7 ± 6.3 and 19.4 ± 6.5 % for ECPsynch. Effects of parity (P<0.01) and parity by treatment (P<0.10) were detected on PRs. In Experiment 2, onset of estrus and time of ovulation were determined in lactating dairy cows submitted to the ECPsynch protocol as described in Experiment 1. Estrus was detected by HeatWatch r and ovulation time determined by ultrasound at 8-h intervals beginning 18 h after onset of estrus or at 48 h after ECP injection. Frequencies of detected estrus and ovulation were 75.7 % (28/37) and 89.2 % (33/37), respectively. Mean intervals to ovulation were 58.5 ± 4.0 h (n=33) after ECP and 27.1 ± 1.1 h (n=28) after onset of estrus. Estrus occurred at 33.6 ± 4.4 h (n=27) after ECP. Percent cows ovulating at an optimal interval of ≥42 h to ≤70 h after ECP was 75.8 % (n=25/33). Synchronization of ovulation and fertility results indicated that ECP can be utilized to induce ovulation for a timed insemination.