Hormone responsible for milk let down

Hormone responsible for milk letdown is Oxytocin.
The oxytocin reflex is also sometimes called the “letdown reflex” or the “milk ejection reflex”. Oxytocin is produced more quickly than prolactin. It makes the milk that is already in the breast flow for the current feed and helps the baby to get the milk easily.
It plays an important role in eliciting milk ejection in dairy cows. Oxytocin is released from the posterior pituitary gland into the blood circulation by various stimuli associated with suckling or milking.