dairy cow vs milk cow

dairy cow

noun
  • A cow bred for the ability to produce large quantities of milk, from which dairy products are made. 

  • A black-and-white varietal of the Porcellio laevis woodlouse. 

milk cow

noun
  • A dairy cow. 

How often have the words dairy cow and milk cow occurred in a corpus of books? (source: Google Ngram Viewer )