Abstract In the present study, effects of Ovsynch-CIDR protocol was compared with the conventional method of two consecutive injections of PGF2 14 days apart, on some reproductive indices and concentrations of E2 and P4 on day1, day 5 and 21 after insemination. All the cows assigned into the study were in their second parity onward and had no history of peri-parturient diseases including: retained placenta, dystocia, lameness, clinical mastitis and metritis. In the nPG group (n=27), on day 40± 5 postpartum, two doses of PG were injected 14 days apart and if after the second injection, cow was inseminated upon showing signs of estrus. In the OC group (n=25), Ovsynch-CIDR protocol along with timed AI was applied. Milk and blood samples were obtained from all the cows on day1, day 5 and 21 post AI. E2 and P4 of the plasma and P4 of the milk samples were measured by ELISA, and reproductive indices were calculated. In case of return to estrus, the program was repeated for up to three times. Conception rate (CR) was lower in the nPG than OC but the difference was not significant (P>0.05). In the second and third AI, CR was higher in OC than nPG (71.42% and 85.71% Vs 23.8% and 61.9% in the OC and nPG, respectively, P<0.05). Number of AI/conception and overall CR were 3.13 & 40.3%, and 2.23 & 55.71% for nPG and OC, respectively (P<0.05). Days open (DO) and predicted calving intervals were 138 & 418 and 106 & 387 for nPG and OC, respectively (P<0.05). Milk P4 was higher on day 5 and 21 in the OC and plasma E2 was lower in OC on day 5 than nPG (P<0.05). Results show that, ovulation synchronization by Ovsynch-CIDR, as compared with two consecutive PG injections reduced DO, increased CR in the 2nd and 3rd inseminations, increased overall CR and reduced number of inseminations per conception. It can not be claimed that results of this experiment might be similar in all the cows of the same herd or other herds because, cows assigned into this study were selected.

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