IDK, they don't add that much to milk as already pointed out earlier. If you are below the recommended serum level you need to put in at least 3k IU to get somewhere. Milk has a max of 85 per 100 ml, so half galon a day would probably be 1.5k IU or less.
Drinking that much milk sure has down sides that are larger than the upside of the vitam D!
G