Keep in mind killing the plant with herbicide just kills the plant, the urisol is still out there and the plants far less visible when dead.
Although I have not needed to clear acres of poison ivy or similar plants on my acre it was far easier, cheaper and very effective to suit up and pull it by hand. The first year I stuffed two forty gallon bags full that weighed about forty pounds each. I have only had it return around two trees and it's easy to spot.
