drop-seed vs smut grass

drop-seed and smut grass both are nouns.

Word	Noun	Adjective	Verb	Adverb
drop-seed	Yes	No	No	No
smut grass	Yes	No	No	No

As nouns, smut grass is a hyponym of drop-seed; that is, smut grass is a word with a more specific, narrower meaning than drop-seed:

drop-seed: a grass of the genus Sporobolus
smut grass: grass native to West Indies but common in southern United States having tufted wiry stems often infested with a dark fungus

drop-seed (noun)	smut grass (noun)
a grass of the genus Sporobolus	grass native to West Indies but common in southern United States having tufted wiry stems often infested with a dark fungus