I remember, growing up in the 40s and 50s, my dad said that Jersey milkcows had the richest milk of any commercial dairy cow. He said that Holsteins were the volume producers, but with the lowest % milk fat. Guernsey was considered the happy compromise. Gufstafson's Dairy
promoted "Golden Guernsey Milk". I don't know if Gufstafson stuck with Guernsey cows.