Lactogenesis refers to the onset of milk secretion, marked by enzymatic and structural differentiation of the mammary alveolar cells. This functional transformation of the mammary gland in preparation for milk synthesis is driven by hormonal changes during pregnancy and early lactation. These hormonal shifts lead to the up- and downregulation of genes responsible for secretory differentiation and activation. Throughout this process, the alveolar cells exhibit ultrastructural changes such as an increase in DNA content, a higher presence of mitochondria, Golgi apparatus, and secretory vesicles. Additionally, microvilli begin to appear and the tight junctions between cells close, signaling further maturation. As secretory activation begins, key enzymes involved in glycolysis, the pentose phosphate pathway, and the synthesis of milk components like fat, protein, and lactose are activated to support the metabolic needs of milk production. This heightened metabolic demand during early lactation places dairy cows under significant metabolic stress, often resulting in a state of negative energy balance (NEB). To adapt, the body prioritizes nutrient allocation through homeostatic and homeorhetic mechanisms. One major adjustment is the development of insulin resistance, which helps redirect glucose toward the mammary glands by boosting liver gluconeogenesis and minimizing glucose use by other tissues. Enhanced fat breakdown (lipolysis) and reduced fat storage (lipogenesis) allow fatty acids to serve as key energy sources and contribute to milk fat synthesis. Concurrently, muscle protein is broken down to supply amino acids for glucose production via gluconeogenesis. Various body systems realign to support these metabolic shifts. The skeletal system undergoes increased bone demineralization to supply minerals needed for milk production. Digestive and renal systems adapt to improve nutrient absorption and retention. Cardiovascular activity also increases, ensuring greater blood supply to the mammary glands. However, elevated glucocorticoid levels and changes in blood metabolites can significantly suppress the immune system, making cows more vulnerable to diseases during this critical period of lactation.