Efficacy of decreasing the dose of GnRH used in a protocol for synchronization of ovulation and timed AI in lactating dairy cows

摘要

237 primiparous and multiparous lactating Holstein cows were randomly assigned to 2 treatment groups. Ovulation was synchronized for cows in the first group using the following treatment schedule: day 0, GnRH 100μg; day 7, 25 mg prostaglandin F2alpha; day 9, 100μg GnRH. The same treatment schedule was used for the second group but only 50μg GnRH was given per injection. All cows underwent a timed insemination 12-18 h after the second GnRH injection. The number of cows ovulating in response to the second GnRH injection (synchronization rate) and pregnancy status 28 and 56 days after insemination were determined by transrectal ultrasonography. The 2 groups did not differ in synchronization rate, double ovulation rate, conception rates at 28 and 56 days, and pregnancy loss from 28 to 56 days. Averaged over all cows, synchronization rate was 84.0%, and double-ovulation rate was 14.1%. Conception rate, calculated for all cows receiving the synchronization treatment, was 41.1 and 34.4% at 28 and 56days respectively after insemination. Corresponding values for only the synchronized cows were 47.6 and 40.1%. For all cows, pregnancy loss from 28 to 56 days was 13.5%, with an attrition rate of 0.5% per day. Estimated savings in hormone costs from using 50 instead of 100μg GnRH per injection for synchronizing ovulation were dollar 6.40 per cow and dollar 20.27 per pregnancy. It is concluded that the reduced dose of GnRH can be used to synchronize ovulation without compromising the efficacy of the synchronizationprotocol.