I think that you have one problem - that is that you have not specified where you wish to hunt or how far you are willing to travel.
From my personal observations - a food plot is only good for about 2 0r 3 years before it needs to be replanted anyways and spraying Round Up on a food plot is not always a bad thing. But the price of the round up would be cost prohibitive. You couldn't buy enough of it to make a difference on a 5 acre field.
I bought a gallon last summer and sprayed twice around the foundation of my house and some weeds growing up out of the sidewalk and it is already 3/4 empty.
You would need a helicopter and a 55 gallon drum of Agent Orange to kill a 5 acre field.