Pasturage is a rather formal, slightly old word for pasture: grass land where cows or sheep eat, the grass itself, or sometimes the right to put animals there to feed. You meet it in farming history, in law, and in older books — “summer pasturage in the hills.” In a shop or a classroom, people will understand you, but they themselves will say pasture or grazing land. It is not wrong; it is just dressed up. Though actually, the tidy dictionary line “pasture” hides a small extra use: pasturage can mean the feeding rights, not only the field.