field corn vs field crop

field corn and field crop both are nouns.

Word	Noun	Adjective	Verb	Adverb
field corn	Yes	No	No	No
field crop	Yes	No	No	No

As nouns, field crop is a hypernym of field corn; that is, field crop is a word with a broader meaning than field corn:

field corn: corn grown primarily for animal feed or market grain
field crop: a crop (other than fruits or vegetables) that is grown for agricultural purposes

Other hypernyms of field corn include corn, Indian corn, maize, Zea mays.

field corn (noun)	field crop (noun)
corn grown primarily for animal feed or market grain	a crop (other than fruits or vegetables) that is grown for agricultural purposes