ricegrass vs cereal grass

ricegrass and cereal grass both are nouns.

Word	Noun	Adjective	Verb	Adverb
ricegrass	Yes	No	No	No
cereal grass	Yes	No	No	No

As nouns, cereal grass is a hypernym of ricegrass; that is, cereal grass is a word with a broader meaning than ricegrass:

ricegrass: any grass of the genus Oryzopsis
cereal grass: grass whose starchy grains are used as food: wheat, rice, rye, oats, maize, buckwheat, millet

Other hypernyms of ricegrass include cereal.

ricegrass (noun)	cereal grass (noun)
any grass of the genus Oryzopsis	grass whose starchy grains are used as food: wheat, rice, rye, oats, maize, buckwheat, millet