RESUMO
Two experiments were conducted to determine the effect of estradiol cypionate (ECP), when incorporated into a conventional GnRH-PGF(2alpha)-GnRH timed artificial insemination protocol (Ovsynch), on systemic estradiol (E(2)), time and incidence of ovulation, luteal development, and conception rate in Holstein cows. Our objective was to determine if administration of 0.25 mg of ECP at the time of the second GnRH injection would effectively synchronize ovulation and increase conception rate. In Experiment 1, lactating Holstein cows (n = 23; 58.7 +/- 1.2 d in milk) were synchronized with PGF(2alpha) (at d -10). Ten days later, Ovsynch was initiated with the administration of 100 mug of GnRH (d 0) followed by PGF(2alpha) on d 7. On d 9, cows were assigned randomly to be treated with either GnRH + 0.25 mg of ECP (OVS-ECP; n = 11) or GnRH and 1 mL of cottonseed oil (OVS-C; n = 12). Ovarian activity was monitored by ultrasonography on d 0, 7, and 9. To determine the time of ovulation, ultrasound examinations were conducted at 12 and 20 h posttreatment and then at least every 3 h until either 36 h posttreatment or ovulation was observed. Blood samples were collected on d 0, 7, 9, and 16 for progesterone analysis. Blood samples also were collected at the time of treatment (d 9, 0 h) and at 6, 12, 20, and 28 h for E(2) analysis. Incidence of ovulation did not differ between treatments. Mean ovulation time relative to the second GnRH administration was similar between treatments. Serum progesterone concentration did not differ between treatments at any time. Serum E(2) concentration was not different at the time of treatment (0 h); however, mean E(2) concentration was greater for the OVS-ECP group at 6 and 12 h after treatment compared with OVS-C. In Experiment 2, lactating dairy cows (n = 333) in 3 commercial herds were randomly assigned to OVS-ECP (n = 169) or OVS-C (n = 164). Cows were inseminated 22 to 24 h posttreatment. Conception rates did not differ between treatments. Estradiol cypionate treatment was successful in increasing serum E(2) when administered at the time of the second dose of GnRH in the Ovsynch protocol. Conception rates, however, were not affected by treatment.
