Fat content from different cattle breeds do differ....as seen at this site:http://www.essortment.com/all/dairiescattleb_rixu.htm

Traditionally, farmers wanted higher fat content,as the fat is separated for other products....like butter, cheese and cream. Years ago, in some areas the milk was of no value....the cream (32% fat) was separated and sold, and the skim milk fed to pigs...Today, the whole milk is sold and separated at dairy plants.

Wondering about goats milk....some see it as superior to cows....you be the judge:
http://askdrsears.com/html/3/t032400.asp