grassland vs hayfield

grassland and hayfield both are nouns.

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

As nouns, hayfield is a hyponym of grassland; that is, hayfield is a word with a more specific, narrower meaning than grassland:

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