silk grass vs rice grass

silk grass and rice grass both are nouns.

Word	Noun	Adjective	Verb	Adverb
silk grass	Yes	No	No	No
rice grass	Yes	No	No	No

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

silk grass: valuable forage grass of dry upland areas and plains of western North America to northern Mexico
rice grass: any grass of the genus Oryzopsis

Other hypernyms of silk grass include ricegrass.

silk grass (noun)	rice grass (noun)
valuable forage grass of dry upland areas and plains of western North America to northern Mexico	any grass of the genus Oryzopsis