silkgrass vs rice grass

silkgrass and rice grass both are nouns.

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

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

silkgrass: 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 silkgrass include ricegrass.

silkgrass (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