Just posting to clarify a few things about cows and milk. First, cows no longer eat grass. Cows ate grass decades ago when every farmer had a few pigs for butchering and a few cows so the farmer could have some milk to drink every day. Back then, milking cows (cows are often "dry", a time when the cow doesn't give milk to prepare the animal to have a strong, healthy offspring), were penned up at night close to the milk barn and then milked once in the morning and once in the evening. During the day, they were put in the pasture to eat grass. Unless you currently buy your milk from some farmer who only has a couple of cows, now your milking cows are either in the barn getting milked, or they are in a "loafing" or a "free stall" barn where they eat, drink and sleep. The food is brought to them, which usually consists of silage (the complete corn stalk is chopped up for feed) and a little bit of roughage, such as hay or alfalfa. A very small amount of ground-up grain may also be given to them, which could be made from the seed of corn or milo.
When that cow is being milked, she doesn't know and couldn't care less if her milk goes to WalMart, Aldi or some other place where the milk is sold for twice the amount. Feed CAN affect the taste of milk, but most dairy farms feed the same feed 365 days a year. However, a sudden introduction of fresh feed or if the small farmer suddenly switches from fermented feed to fresh feed, can definitely change the taste of milk.
When you taste the difference of your milk between the brands, it's because of how the milk was treated or stored after the milk comes out of the cow. We have a local mom-and-pop grocery store that sells milk that will be good until the the date printed on the carton and not any later. Some other milk in my area sold by WM or Aldi's can taste good a week after that date.
Yes, I grew up on a dairy farm.