wheat vs food grain

wheat and food grain both are nouns.

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

As nouns, food grain is a hypernym of wheat; that is, food grain is a word with a broader meaning than wheat:

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

Other hypernyms of wheat include cereal, grain.

wheat (noun)	food grain (noun)
a variable yellow tint; dull yellow, often diluted with white	foodstuff prepared from the starchy grains of cereal grasses
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