Dairy cattle become pregnant by fertilization during a lactation period. Five months after fertilization, the cattle begin to actively secrete hormones, such as estrogen, from their placentas and milk secretion decreases. As a result, part of the mammary gland tissue and cells start to regress.
As mammary glands in mother cattle regress, milk productivity is decreased while the fetal cows grow. Dairy farmers stop milking at an appropriate time in the precalving period in order to induce the mammary gland to enter the dry period. This will facilitate smooth parturition by dairy cattle and resumption of postcalving lactation.
During the dry period, new mammary tissue is generated and differentiated for suckling and feeding to newborn calves. This generation and differentiation occurs due to regression of the old mammary epithelium in the mucosal mammary tissue by apoptosis and the replacement ingression of various blood cells. When the generation and differentiation begins, mammary cells and tissues comprise extremely diverse constituent cells, including granulocytes, monocytes, macrophages, dendritic cells, NK cells, T-cells, B-cells and mast cells. Phagocytic cells such as granulocytes and macrophages are distributed in the mammary tissue and milk. Lymphocytes, such as helper CD4+ T-cells derived from the bloodstream and antibody-producing B-cells, are also distributed (Vet. Immunol. & Immunopathol., Vol. 65, 51–61, 1998; J. Dairy Sci., Vol. 82, 1459–1464, 1999). Thus, mammary gland resistance to infection is increased in the dry period.
The current method of inducing dairy cows to enter the dry period is to stop milking the cows in order to increase pressure within the mammary glands. However, in reality, some mammary glands of lactating cows struggle in the smooth transition to the dry period even with of the performance of this method. In such case, the cows encounter the problem that exogenous bacteria may easily infect the mammary glands resulting in an increased incidence of mastitis.
If the transition is delayed and troubled, the mammary gland of mother cows may be retarded. Further, when the mother cow raises calf, the calf may also encounter problems such as defective development, or be affected by infectious diarrhea caused by various bacteria or viruses.
These problems are serious to dairy farmers, and the earliest possible resolution of these problems is desired. However, no useful means to solve them has been found.
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