LACTATION is defined as the process of producing and secreting milk from the mammary gland. All mammals provide nutrients for their newborns in the form of milk from the mammary glands. Dairy cows do such a good job of it that they could feed more than 10 calves. Since they usually only have one calf, humans can harvest the rest, pasteurize it, homogenize it, package it, and sell it in the store for you to drink.
MILK LET DOWN is the process by which a cow will release the milk from her mammary tissues where milk is secreted. This process occurs after the mammary gland is stimulated. Initially, the stimulation comes from the calf nudging the udder, but calves are usually removed from the presence of their mother within one hour and, thus, get replaced by the dairy producer, massaging the udder while it is cleaned off for the milking machine and forestripping milk from quarters.
Dick Wallace, Dairy Extension Veterinarian, University of Illinois - Illini DairyNET
