Kraevskіy A., Dopa , V., Chekan , A. and Musiienko, Y. 1. Age structure of fertilization of heifers and its influence on the frequency of complication of calving in first-calf cow and their culling from the herd . Bulletin of Sumy National Agrarian University. The series: Veterinary Medicine. 1 (48) (1), 23-31. DOI:https://doi.org/10.32845/bsnau.vet.2020.1.4.