Predictable ovulation time and increased serum estradiol concentration achieved by GnRH administration at 56-hour proestrus in lactating dairy cows

Abstract

The feasibility of modified ShortSynch protocols was evaluated to determine the effects of prolonged proestrus on elevation of circulating estradiol (E2) levels and synchronous ovulation in lactating dairy cows. Cows (n = 15) ranging between 50 and 70 days postpartum were pre-synchronized by a modified ShortSynch protocol (375 mg PGF-1 day-250 mg of PGF-1 day-GnRH) to induce ovulation. After the initial ovulation, a switch-back design (stages 1 and 2, respectively) was used to compare the effects of GnRH administration at 48 versus 56 h after PGF (length of proestrus) in the same cow with modified ShortSynch protocols. Seven to eight days after ovulation, the first PGF treatment (defined as Hour 0) was administered to the cows, with a second PGF given at Hour 24. Cows were randomly allocated to receive GnRH either at Hour 48 (G48 group) or Hour 56 (G56 group) in stage 1 and vice versa in stage 2. All cows showed complete luteal regression and 93.3% (28/30) ovulated between 28 and 32 hours after GnRH administration. Extending the interval of proestrus by eight hours had an effect (P < 0.05) on preovulatory E2 levels, with the E2 concentration for G56 higher than that for G48 at GnRH administration (41.0 ± 3.3 vs. 33.3 ± 3.9 pg/mL; P = 0.02). The present study suggests that modified ShortSynch protocols could help achieve high synchronization rates with a predictable time of ovulation after GnRH administration, in addition to promoting higher circulating E2 levels with the extension of proestrus from 48 to 56 hours.