milker vs milk cow

milker and milk cow both are nouns.

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

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

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

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