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Ovsynch is a protocol routinely used for oestrus synchronization with the best results obtained when it is started in the midluteal phase of the oestrous cycle. The best way to achieve this phase is to apply presynchronization protocols. The aim of this study was to compare the success of two combined presynchronization protocols with Ovsynch, according to conception rates and progesterone concentration, in synchronization and resynchronization. The research was conducted on a dairy farm in Croatia, on 60 cows divided into three equal groups. Group A was presynchronized with GnRH and PGF2α nine and two days before starting Ovsynch. Group B was presynchronized with just one application of PGF2α two days before Ovsynch. The third group C served as a control, and was subjected to Ovsynch only. All cows were artificially inseminated at a fixed time. Twenty-three days after TAI, GnRH was applied to all cows, as the beginning of Ovsynch in resynchronization. Seven days later all the cows were examined for pregnancy with ultrasound. Those diagnosed as non-pregnant, continued with Ovsynch and TAI. Blood samples were collected for progesterone analysis. The results indicate that cows subjected to presynchronization had a better pregnancy rate compared to cows subjected to Ovsynch only (A 29.41%, B 47.37% and C 21.05%). In resynchronization, the conception rate did not differ between the groups, and was around 30%. Progesterone concentrations differed significantly between group C and groups A and B (P<0.01) at the start of Ovsynch in synchronization (Day 0), indicating the more effective homogenization of folliculogenesis and synchronization of ovulation. Both the combined synchronization protocols used in this research are original. Due to the results obtained, their application should be evaluated further.

Progesterone concentration and conception rates after three different synchronization protocols in dairy cows