food grain vs buckwheat

food grain and buckwheat both are nouns.

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

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

food grain (noun)	buckwheat (noun)
foodstuff prepared from the starchy grains of cereal grasses	grain ground into flour
	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