9 Estrogen by itself generally stimulates duct development of the mammary glands, while estrogen and progesterone together cause lobulo-alveolar growth. In the goat, however, estrogen alone will initiate abnormal development of the mammary, producing dilated alveoli and even milk production in some cases. High levels of estrogen in circulation have a negative effect on feed consumption and milk production in goats. Estrogen and progesterone are required both for the complete development of the mammary system, along with the other pituitary hormones. Large amounts of prolactin are to be used just prior to parturition in response to the higher levels of estrogen and progesterone at this time.
Prolactin matures the alveolar cells to a functional condition.
STH and thyroxine, also exert an influence on the amount of milk produced. Small injections of estradioltestosterone produce a colostrum-like milk in goats, while large doses produce a watery, clotted milk secretion. The hormone of most interest is oxytocin. It is responsible for milk let-down, causing the myoepithelial cells that surround the alveoli to contract, forcing the milk out into the ducts of the udder. Oxytocin release is initiated in several ways, the most natural being nursing.