Final answer:
The hormone that causes milk letdown and release in cows is oxytocin, which stimulates the contraction of myoepithelial cells in the mammary gland. Prolactin is also critical as it signals the mammary glands to produce milk.
Step-by-step explanation:
The hormone responsible for milk letdown and release when a cow is milked is called oxytocin. This hormone is released by the posterior pituitary and it stimulates the myoepithelial cells located around the milk-producing alveoli to contract. When these cells contract, milk is expelled from the alveoli, passes through the lactiferous ducts, collects in the lactiferous sinuses, and finally discharges through the nipple pores. It is interesting to note that this process is part of a positive feedback loop triggered by the suckling of an infant, which sends nerve impulses to the hypothalamus to release oxytocin.
Another essential hormone in the process of lactation is prolactin, which is produced by the anterior pituitary gland. Prolactin is responsible for stimulating the mammary glands to produce milk. Nonetheless, the actual ejection of milk, commonly known as the milk letdown reflex, is primarily facilitated by oxytocin.