rice vs food grain

rice and food grain both are nouns.

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

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

Other hypernyms of rice include starches, cereal, grain.

rice (noun)	food grain (noun)
grains used as food either unpolished or more often polished	foodstuff prepared from the starchy grains of cereal grasses
annual or perennial rhizomatous marsh grasses; seed used for food; straw used for paper

rice (verb)	food grain (verb)
sieve so that it becomes the consistency of rice