pasture land vs grassland

pasture land and grassland both are nouns.

Word	Noun	Adjective	Verb	Adverb
pasture land	Yes	No	No	No
grassland	Yes	No	No	No

As nouns, grassland is a hypernym of pasture land; that is, grassland is a word with a broader meaning than pasture land:

pasture land: a field covered with grass or herbage and suitable for grazing by livestock
grassland: land where grass or grasslike vegetation grows and is the dominant form of plant life

pasture land (noun)	grassland (noun)
a field covered with grass or herbage and suitable for grazing by livestock	land where grass or grasslike vegetation grows and is the dominant form of plant life