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Journal of Animal and Veterinary Advances
Year: 2007  |  Volume: 6  |  Issue: 11  |  Page No.: 1328 - 1332

The Efficacy of a Reduced Dose of Gonadotropin Releasing Hormone on the Time and Incidence of Ovulation in Holstein Cows

A. Ahmadzadeh, R. Manzo, C.B. Sellars and J.C. Dalton    

Abstract: The objectives were to determine the effect of a reduced dose of GnRH (50 µg) on the incidence and time of ovulation following an ovulation synchronization protocol (Ovsynch) in lactating Holstein cows. Fifty-five days postpartum (14 day) cows received (i.m.) 25 mg PGF2 . Fourteen days later (0 day) cows were assigned to1 of 2 treatment groups. Cows in Group 1 received half of the recommended dose of GnRH (HD; n = 17) and received 50 µg of GnRH on d 0, PGF2 (25 mg) on day 7 and GnRH (50 µg) on day 9. Cows in Group 2 were assigned to a full dose of GnRH (100 µg; FD; n = 17) and were synchronized using the same protocol. Ovarian activities were monitored by ultrasonography on day 0, 7 and 9 and also at 12 h, 20 h and then every 3 h until 39 h after the second GnRH injection. Blood samples were collected on day 7 and analyzed for serum Progesterone (P4). Mean BW was not different between groups. On day 7, mean serum P4 was above 1 ng mL 1 and did not differ between the groups. On day 9, concentrations of P4 decreased to below 1 ng mL 1 and did not differ between groups. Mean diameter of the ovulatory follicles in HD and FD were similar. Incidence of ovulation and mean ovulation time relative to the second GnRH did not differ between the groups. None of the cows exhibited estrus during the study period. Cows that ovulated developed a Corpus Luteum (CL) which was visualized using ultrasonography 7 d after ovulation. These results indicated that a reduced dose of GnRH did not affect ovulation time relative to the second GnRH injection and did not compromise the incidence of ovulation and luteal development when used in an Ovsynch breeding protocol.

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