Lactation
This is the secretion and ejection of milk from the mammary glands. A principle hormone in promoting milk synthesis and secretion is prolactin. It is secreted by the anterior pituitary. As prolactin levels increase during pregnancy, no milk is secreted due to the inhibitory effects of progesterone on prolactin. The principle stimulant for prolactin secretion is the suckling of the foal. Stretch receptors in the teat send impulses to the hypothalamus. In response, secretion of oxytocin from the posterior pituitary increases. Carried by the blood stream to the mammary glands, oxytocin stimulates contraction of smooth muscle cells surrounding the glandular cells and ducts. This moves the milk form the alveoli of the glands to the ducts for the foal. This process is termed milk ejection or let-down. Other stimuli can trigger oxytocin release such as hearing a foal’s whinny or the foal touching the mare’s udder. Colostrum is secreted late in pregnancy and the first few days after birth and is a cloudy fluid not as nutritious as milk but serves adequately until the appearance of true milk on the 4 th day. Colostrum& maternal milk contain important antibodies that protect the foal during the first few months of life. Lactation often blocks ovarian cycles for the first few months following delivery if the frequency is high enough.
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