Abstract
The pituitary response, ovulation synchronization rate (OSR), embryo survival rate (ESR), and pregnancy rate (PR) were compared in dairy cows treated with Ovsynch protocol with two different intervals between the prostaglandin F2α(PGF2α) and 2nd GnRH injections. A total of 27 Holstein cows were injected with a GnRH analog on day0 and a PGF2α analog on day7. A 2nd GnRH injection was administered 30 hr after the PG injection to 13 animals (30-hr group), and 48 hr after the PG injection to 14 animals (48-hr group). Timed artificial insemination (TAI) was performed 16 hr after the 2nd GnRH, and the ovaries were examined for ovulation 24 hr after TAI. Blood samples were obtained during the program and plasma progesterone, estradiol-17β and luteinizing hormone (LH) concentrations determined. ESR was calculated from early pregnancy factor activity in blood taken 48 hr after TAI. OSR, ESR and PR were respectively 84.6%, 53.8% and 46.2% in the 30-hr group and 85.7%, 61.5% and 50.0% in the 48-hr group, with no significant difference between the two groups. Although the basal LH level (at the 2nd GnRH) in the 48-group was significantly higher (P<0.05) than in the 30-hr group, no significant difference in LH response to GnRH was found. It was concluded that a 30 hr interval between PGF2α and 2nd GnRH during an Ovsynch program has a similar effect to that of a 48-hr interval, and that fertilization failure or early embryonic death within 48 hr post-TAI is involved in the majority of failed pregnancies.
