Aim of the study To investigate the dynamic changes of ovaries and their functional derivatives, progesterone concentration and estrous cycle synchronization during different breeding periods in beef heifers and cows of different breeds. Objectives of the study: 1. To examine and compare the parameters of the estrous cycle of heifers of different beef breeds by assessing the dynamic changes of the ovaries, their functional derivatives and progesterone concentration. 2. To examine and compare the parameters of the estrous cycle of cows of different beef breeds by assessing the postpartum period, the dynamic changes of the ovaries, their functional derivatives and progesterone concentration. 3. To examine and evaluate the efficiency of estrous cycle synchronization programs in beef heifers and cows while assessing changes in ovarian follicles and corpus luteum and progesterone concentration. 4. To evaluate the analysis of gene types, which encode proteins involved in reproductive processes, and their possible association with reproductive traits. Novelty and practical importance of the study Our work allowed to evaluate the latest synchronization technologies (presynchronization, synchronization and timed artificial insemination) while using veterinary medications registered in Lithuania for heifers and cows of various beef breeds. The practical side of our work and the results provide preconditions for the application and adaptation of these biotechnologies in the conditions of our country, while aiming to achieve optimal reproduction results of beef cattle herds. In the scientific novelty,…