dairy cow vs milker

dairy cow and milker both are nouns.

Word	Noun	Adjective	Verb	Adverb
dairy cow	Yes	No	No	No
milker	Yes	No	No	No

As nouns, dairy cow and milker are synonyms defined as:

Other synonyms of dairy cow include dairy cattle, milch cow, milcher, milk cow.

dairy cow (noun)	milker (noun)
cattle that are reared for their milk	cattle that are reared for their milk