food grain vs wild rice

food grain and wild rice both are nouns.

Word	Noun	Adjective	Verb	Adverb
food grain	Yes	No	No	No
wild rice	Yes	No	No	No

As nouns, wild rice is a hyponym of food grain; that is, wild rice is a word with a more specific, narrower meaning than food grain:

food grain (noun)	wild rice (noun)
foodstuff prepared from the starchy grains of cereal grasses	grains of aquatic grass of North America
	perennial aquatic grass of North America bearing grain used for food