dairy cow vs milking machine

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. 

milking machine

noun
  • A machine for milking cows. 

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