The specificity of the digestive system of ruminants is manifested in the structure of the digestive system that is the existence of the foregut, of which the most important is the rumen. A key factor for the proper growth and development of calves is knowledge of the anatomy and physiology of the digestive system of ruminants, the method and type of feeding adapted to the age of the calf and breeding technology. The calf is born as a non-functional ruminant with a well-developed rennet. A key step for development is the intake of a sufficient amount of colostrum by which the calf acquires the necessary antibodies. The intake of solid food adapted to the age of the calf encourages the development of a micropopulation of rumen consisting of bacteria, fungi and protozoa that form a symbiosis with the host and degrade and form substances that eventually resorb and meet the needs of animals for nutrients and energy. In addition to feeding, housing technology and zoohygienic conditions are some of the more important factors that indirectly affect the development of the rumen, and in certain cases can lead to disease and lag in calf growth.