rice grass vs silkgrass

rice grass and silkgrass both are nouns.

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

As nouns, silkgrass is a hyponym of rice grass; that is, silkgrass is a word with a more specific, narrower meaning than rice grass:

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

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