food grain vs wheat

food grain and wheat both are nouns.

Word	Noun	Adjective	Verb	Adverb
food grain	Yes	No	No	No
wheat	Yes	No	No	No

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

food grain: foodstuff prepared from the starchy grains of cereal grasses
wheat: grains of common wheat; sometimes cooked whole or cracked as cereal; usually ground into flour

food grain (noun)	wheat (noun)
foodstuff prepared from the starchy grains of cereal grasses	a variable yellow tint; dull yellow, often diluted with white
	grains of common wheat; sometimes cooked whole or cracked as cereal; usually ground into flour
	annual or biennial grass having erect flower spikes and light brown grains