Aim: In this study, the effect of ovarian functions on estrus synchronization and pregnancy rates in Brown Swiss heifers and non-lactating cows synchronized with a modified ovsynch protocol was investigated.

Materials and Methods: The estrous cycle of heifers (n=60) and cows (n=27) was synchronized with two injections of 150 mcg d-cloprostenol (PGF_2α) 14 days apart. Ovulation was induced with the injection of 50 mcg lesirelin acetate (GnRH) at 48 hour after the second PGF_2α, and insemination (AI) was performed between 16-20 h following the last injection. Corpus luteum and follicle diameters were measured on days of PGF_2α and GnRH injection with ultrasound. Moreover, blood samples were collected to assess plasma progesterone concentration on days of PGF_2α injection and on AI day (day 0) and subsequently on 7, 14 and 21 days. Pregnancy was checked on day 28 after AI.

Results: Synchronization and pregnancy rates were found 87.4% and 36.8%, respectively. The most important factor reducing the efficacy of protocol during synchronization period was no luteal function in 6.89% of cows at time of the second PGF_2α injection. The affecting negatively on pregnancy rates in synchronized cows was determined two main factors. 1- Lack of luteal function on day 7 post-insemination (11.84% of synchronized). 2- Possible early embryonic deaths (11.84% of synchronized).

Conclusions: It was concluded that the ovulation can be synchronized with modified ovsynch protocol, consisting of two PGF_2α and one GnRH injection, and could be achieved to acceptable pregnancy rate in Brown Swiss heifers and non-lactating cows.