My chickens love milk and almost any kind of dairy product, like unflavored or vanilla yogurt, cottage cheese, sour cream, or regular shredded cheese.
My husband and I do not drink an entire gallon of milk before it starts to go bad, but that is no problem because the chickens will drink any cow’s milk that is too old for us to drink.
I learned alot from a professional farmer who produced excellent organic raw milk, organic eggs,organic butter and cheese made from raw milk, and kefir. At one time, she was the only USDA certified producer of such foodstuffs anywhere in west Tennessee.
From her, I found out that milk is safe to drink by humans and chickens long after the milk sours. I used to throw old milk away when it got old and began to sour, but she pointed out to me that our great great grandparents - who lacked refrigeration - used to drink sour milk and even developed lots of time treasured recipes to use sour milk. It was only in the last century - with the advent of electric refrigerators - that most of us began to limit ourselves to fresh, still sweet milk.
I guess I am spoiled, because I still don’t want to drink the older milk.
But that is OK by my hens - they love that older milk.