What hormone is primarily responsible for milk letdown in dairy cows?

The hormone primarily responsible for milk letdown in dairy cows is oxytocin. During the milking process, when stimulation occurs—such as the calf suckling or the application of a milking machine—oxytocin is released from the posterior pituitary gland. This hormone plays a crucial role by causing the contraction of myoepithelial cells surrounding the milk alveoli in the udder, which helps to move milk into the teat cistern and ultimately allows for its release during milking.

While estrogen and progesterone are important hormones involved in reproductive cycles and mammary development, they do not directly cause the milk letdown reflex. Testosterone, predominantly a male hormone, is not involved in the milk production or letdown mechanisms in cows. Understanding these hormonal roles is essential for effective dairy management practices, particularly in optimizing milk production and ensuring cow comfort during milking.