The formation of colostrum in the pregnant cow is initiated about 3-4 weeks before parturition when a limited amount of fluid containing small amounts of growth factors and other transforming substances is released into the developing mammary tissue. The process is regulated by a series of other hormones, one of the most important being progesterone, which attaches to special receptors on the cells lining the mammary gland and prevents them from secreting any fluid into the gland during most of pregnancy. About two weeks before birth, these substances influence the appearance of specific receptors on the surface of the cells lining the mammary gland that will facilitate the transfer of materials from the mother's blood into the gland, including the immunoglobulins (antibodies) necessary to convey passive immunity to the calf after birth and various hormones and growth promoters required to induce and support development of the newborn calf.
About 2 days before birth, the hormonal balance begins to shift, initiating the production of copious secretions and switching on the ability of cells in the mammary tissue to synthesize various substances, including lactose. At birth, when the placenta is eliminated, progesterone levels fall dramatically in the mother and its inhibitory control of the secretions is removed. Simultaneously, a protein-based substance develops in the cells lining the mammary gland that essentially blocks any further transfer of substances from the mother's blood into the gland. The composition of the fluid in the mammary gland at birth is that of true colostrum and reflects the functional changes that have occurred in the gland up to that time; it a) has a high protein concentration, most of which is IgG; b) contains the highest concentration of growth promoters, other hormones and additional metabolically active substances; c) is low in lactose content; and d) is rich in milk fat.
After the mother cow delivers the calf, the removal of fluid from the mammary gland creates an abundant amount of secretion from the mammary gland. This replacement fluid is called transitional milk and will be different from the original fluid secreted from the mammary gland at birth. Another reason this replacement fluid is different from the original fluid is after birth the mother cow begins to reabsorb the fluid that is in the mammary gland. The highest quality of colostrum is collected in the first milking within six hours of the calf's birth. The first milking contains the highest concentration of biologically active substances and nutrients.
While the colostrum changes its composition, the events complement the events of the newborn baby calf. When the calf is born, its stomach lining has very few enzymes to break down ingested proteins and does not make any acid.
In the first milking, the colostrum passes through the the stomach and the intestine of the calf without being broken down. Between 6-8 hours after birth, the calf begins to absorb and transport biologically active substances into the bloodstream. Then the stomach begins to acidify as well as enzymes begin to appear. The calf is born with lymphoid tissue that is well-developed which absorbs many biological active substances when the calf milks.