ricegrass vs silk grass

ricegrass and silk grass both are nouns.

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

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

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

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