I saw some more posts on raw milk under the introductory form and instead of cluttering up his thread I decided to post here.
In california there are two main raw milk suppliers, organic pastures, and claravale dairy. I have spoken to the owners in person and in e-mail and they mention feeding their cows something like 5lbs of corn a day. They made it sounds like 100% grass fed milk does not really exist in any quantity for sale, and if it did the taste would be terrible. He goes on to mention certain times of the year when the cows eat more pasture than normal, people complain about the taste of the milk every year.
Not sure if anyone realizes this/cares/or if this is just a farm-by-farm issue, but I found it interesting, being that 100% grass fed meats are highly regarded here.