cow pasture vs pastureland

cow pasture and pastureland both are nouns.

Word	Noun	Adjective	Verb	Adverb
cow pasture	Yes	No	No	No
pastureland	Yes	No	No	No

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

cow pasture: a pasture for cows
pastureland: a field covered with grass or herbage and suitable for grazing by livestock

Other hypernyms of cow pasture include grazing land, lea, ley, pasture, pasture land.

cow pasture (noun)	pastureland (noun)
a pasture for cows	a field covered with grass or herbage and suitable for grazing by livestock