These weeds may not be really a problem. Are you useing the 5 acres for a crop that you harvest? Many plants people consider weeds are actually wonderful sources of pollen and nectar for bees. Also, since it's such a large area, it would be very hard to control the whole area. Seeds will still be blowing in from other areas before you could control part of it. If you really MUST be rid of the weeds, and do not already have plans for what to plant there, maybe you could plant something that will take over stronger than the weeds.