Lactation in mammals is a biological process designed to provide nourishment for offspring. The act of calving initiates the body’s shift from gestation to milk production. However, modern dairy management uses hormone treatments to bypass this natural requirement, successfully inducing lactation in cows that have not recently been pregnant. This ability allows high-producing animals with reproductive issues to remain productive within the dairy herd.
The Natural Trigger for Milk Production
The initiation of milk synthesis, known as lactogenesis, is a hormonal event spanning pregnancy and calving. During gestation, hormones like estrogen stimulate ductal growth, and estrogen combined with progesterone regulates the formation of milk-producing structures called alveoli. This development prepares the udder, but high levels of progesterone circulating during pregnancy actively inhibit the full switch to milk production.
The trigger for milk flow occurs immediately following the delivery of the calf. When the placenta is expelled, progesterone levels drop sharply, removing the primary inhibitory block on the mammary tissue. This abrupt change allows other lactogenic hormones, such as prolactin and glucocorticoids, to exert their full effect. Prolactin, released from the pituitary gland, stimulates the alveolar cells to begin synthesizing the components of milk.
Hormonal Protocols for Induced Lactation
Milk production can be initiated artificially by replicating the hormonal environment of late pregnancy and calving. This process, known as Artificial Induction of Lactation (AIL), involves a multi-week protocol of administering exogenous hormones. The objective is to stimulate the development of the alveolar structures while suppressing the natural reproductive cycle.
The protocol begins by injecting a combination of steroid hormones, most commonly 17\(\beta\)-estradiol and progesterone, over about seven days. These hormones mimic the high levels present during the final stages of pregnancy, promoting mammary gland tissue development. Administration of these steroid hormones is then stopped to simulate the sudden hormonal drop that occurs after the expulsion of the placenta.
This is followed by agents that trigger the final switch to milk synthesis, simulating the hormonal environment of calving. Glucocorticoids, such as dexamethasone or flumetasone, are often used, acting synergistically with prolactin. Some protocols also include prolactin-releasing agents like reserpine or the administration of bovine somatotropin (bST) to enhance milk yield. Milking usually begins around 21 to 23 days from the start of the treatment.
Sustaining Milk Flow Through Management
Once milk production is successfully initiated, the primary mechanism for maintaining the flow shifts from hormonal initiation to physical stimulus. Regular removal of milk from the udder sustains the lactation cycle. This is governed by a neuroendocrine reflex known as the milk letdown reflex.
Stimulation of the teats during milking sends a neural signal to the cow’s brain, prompting the release of oxytocin from the posterior pituitary gland. Oxytocin travels through the bloodstream to the udder, causing the myoepithelial cells surrounding the alveoli to contract. This contraction forces milk into the cisterns where it can be extracted. This reflex is short-lived, making timely and complete udder emptying important.
The physical presence of milk within the alveolar space acts as a local inhibitor of further production. This inhibition is mediated by a small protein in the milk known as Feedback Inhibitor of Lactation. Emptying the udder completely and frequently removes this inhibitor, signaling to the mammary cells that milk production should continue. Dairy management practices, such as milking two or three times daily, leverage this feedback mechanism, maintaining the high-volume output necessary for commercial production.