silk grass vs ricegrass

silk grass and ricegrass both are nouns.

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

As nouns, ricegrass is a hypernym of silk grass; that is, ricegrass 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
ricegrass: any grass of the genus Oryzopsis

Other hypernyms of silk grass include rice grass.

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