hayfield vs grassland

hayfield and grassland both are nouns.

Word	Noun	Adjective	Verb	Adverb
hayfield	Yes	No	No	No
grassland	Yes	No	No	No

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

hayfield: a piece of land covered or mostly covered with grass; a field where grass or alfalfa are grown to be made into hay
grassland: land where grass or grasslike vegetation grows and is the dominant form of plant life

hayfield (noun)	grassland (noun)
a piece of land covered or mostly covered with grass; a field where grass or alfalfa are grown to be made into hay	land where grass or grasslike vegetation grows and is the dominant form of plant life