Milk-producing mammals were an important part of early agriculture in the world. Goats were among our earliest domesticated animals, first adapted in western Asia from wild forms about 10,000 to 11,000 years ago. Cattle were domesticated in the eastern Sahara by no later than 9,000 years ago. We surmise that at least one primary reason for this process was to make a source of meat easier to get than by hunting. But domestic animals also are good for milk and milk products like cheese and yoghurt (part of what V.G. Childe and Andrew Sherratt once called the Secondary Products Revolution) 👍 So―when did dairying first start and how do we know that? 
One of the significant contributors to the dairy industry from the private sector was Dr. Charles E. North, a physician, public health officer, inventor, and agricultural scientist. He was a leader in gaining public acceptance of laws around pasteurized milk, as well as authored many articles and reports related to the growth of the dairy industry. One of Dr. North’s most significant achievements was the creation of a system of sanitation that helped any farm produce dependably clean milk. The simple, yet effective system comprised of 6 requirements: healthy cows, careful grooming of livestock, clean hands and clean clothing, clean and dust-free barns, thoroughly washed sterilized milking utensils, and prompt and effective cooling of the milk. (modified by Randol Vanhorn on May 30, 2021) 
Specialcollections.nal.usda.gov goes on to describe that reed was featured in an article about ice cream and its importance to the dairy industry. The story, Ice Cream-An American Institution, was printed on the back cover of a Pennsylvania Railroad Dining Car Menu from 1930 (shown below). According to Reed, there were approximately 4,000 ice cream manufacturers in the United States in 1928. These manufacturers produced about 348,000,000 gallons of ice cream. The three gallon per capita consumption required about 6 billion pounds of milk, the product of 1,333,000 dairy cows. This placed the production of milk for ice cream almost equal to the quantity used in the manufacture of cheese and greater than the quantity used in the manufacture of condensed and evaporated milk. (a big thank you to Jermika Dotson after pointing this out). 
For 60 years the milk boards provided a reliable service. Tankers arrived daily to collect the milk then the milk cheque arrived promptly every month. To some extent farmers traded short term profit opportunities for long term stability and it worked well. The dairy farm I spentpentpent my time on as a child in the 50 s had 12 cows and managed to support three families. Dairying prospered under the aegis of the MMBs. In the early 70 s when I started work as an agricultural advisor in SW Scotland farmers worried about their calving index ,silage quality and mastitis cell count, all technical stuff. The milk price was rarely mentioned, it was OK.