Where are you reading that sour "skim" or "dried" milk is good for poultry?
From what I understand, sour milk is another way of saying clabbered milk. To make clabbered milk, you need raw, unprocessed milk. It needs to be raw because you need the beneficial bacteria in order for the milk to clabber- or sour- rather than spoil.
I've actually tried this. I put a jar of raw milk and a jar of pasteurized milk on the counter and let them sit. They both sour, but the processed milk is putrid & spoiled. The raw milk has soured, so it's more like a yogurt taste. It won't make you sick if you eat it, it's actually good for you. You could probably use a yogurt maker. I actually prefer home made yogurt to the clabbered milk for my own use.
Those beneficial bacteria in raw milk are what's good for the poultry, or any other animal. You can add lemon juice or buttermilk to processed milk to make it sour but it won't have the same health benefits from the good bacteria.
There are people out there that are fanatical about raw milk, but I'm really not one of those. I'm just someone who milks her cows, drinks it raw, didn't die and actually experienced some health benefits from it. I think this is one of those things that the old timers did. They fed their chickens milk from their own cows or goats and the chickens got all the beneficial bacteria etc, from it. Some of the new chicken feeds have these same ingredients added to them.
I think if you try to recreate milk for them from heavily processed milk, it's not going to have the same result. You would be better off finding a source of raw milk from a trustworthy, small farmer.
