Aim of study was to determine relationship between the energy status of cows, expressed through different milk fat to protein ratio values (FPR<1.0, 1.0-1.3 and >1.3), and their reproductive performances (interval from calving to first artificial insemination; calving to conception interval; insemination index; pregnancy duration; intercalving interval) during succesive stages of lactation (day 15 th to 45 th ; day 46 th -75 th ; day 76 th -105 th ; day 106 th to 135 th of lactation). The study included a total of 350 fresh calved Holstein cows (121 primiparous, 115 secundiparous and 114 cows which calved three or more times), kept and fed in usual farm conditions. Milk samples were taken during morning milking, and FPR value was calculated from concentrations of milk fat and protein for all milk samples. Reproductive parameters were calculated from farm data. Values of all reproductive parameters, except pregnancy duration, had generally increasing trend in all groups during successive stages of lactation. During successive stages of lactation, cows with optimal FPR values had generally better reproductive performances, compared to cows with increased or decreased FPR values, which indicates importance of FPR monitoring during early lactation for timely detection of cows predisposed for poor reproductive results.

During the past few decades, continued genetic progress of milk production, coupled with new reproductive technologies performed in high-producing dairy cows, has led to significant decrease in dairy cows fertility all over the world. Delayed postpartum anoestrus is one of the major causes of decreasing fertility and economic losses in intensive milk production. The aim of this paper is to review the causes and possibility for reducing its impact on subsequent reproductive performance in dairy cows.