Health Network News

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.

The hormone balance in the mother cow begins to change about 2 days prior to the birth of the calf. This hormone change initiates secretions and combines substances together. After the baby calf is born and the placenta is delivered, progesterone levels in the mother drop. The blocking of secretions by progesterone is eliminated; a protein-based substance is created in the mammary gland cells and transfers of substances from the mother’s blood into the mammary gland are stopped. The fluid in the mother cow found in the mammary gland at birth is called colostrum. It contains high amounts of hormones, metabolically active substances and growth promoters. Colostrum also has a high concentration of protein, is rich in milk fat and low in its lactose content.

After birth, one of the most influential factors on the composition of subsequent secretions is physical removal of the fluid from the mammary gland. The removal of even small quantities of fluid triggers the production of copious amounts of secretion from the cells in the mammary gland. Since the transfer of biologically-active substances from the mother’s blood is blocked, replacement fluid will contain primarily substances synthesized by cells in the mammary gland and, thus, will be of a different composition than the fluid originally contained in the mammary gland at birth. The fluid expressed at this time is known as transitional milk. This is further complicated by the fact that the basic composition of the colostrum changes after birth due to maternal reabsorption and does so rapidly beginning at six hours, as can be seen from the table below.10 Thus, the highest quality bovine colostrum, containing the maximum concentration of biologically active substances, is collected in a single milking during the first six hours after parturition.

While the colostrum in the mother cow are changing quickly after the birth of the calf, the baby calf is undergoing a number of changes in its body. Within the first six hours of life, the calf’s stomach lining does not produce any acid and there are a limited number of enzymes which break down proteins that are ingested.

When the calf ingests the first colostrum milking, the enzymes travel through the stomach and intestines without having been broken down. After 6-8 hours after birth, enzymes in the calf’s stomach begin to appear in addition to the stomach beginning to acidify.