buckwheat vs food grain

buckwheat and food grain both are nouns.

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

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

Other hypernyms of buckwheat include cereal, grain.

buckwheat (noun)	food grain (noun)
grain ground into flour	foodstuff prepared from the starchy grains of cereal grasses
a member of the genus Fagopyrum; annual Asian plant with clusters of small pinkish white flowers and small edible triangular seeds which are used whole or ground into flour