Depending on the species of weeds, Glyphosate (the chemical name of Roundup) is probably the best option. It usually works best if you use fire first to remove the duff (dead plant material) from the field (check out GDTV 20 for an overview of prescribed fire). This allows the herbicide to make better contact with the fresh vegetation that will grow after the burn. If a burn is not an option, mowing or even disking is the next best option. No matter what option you select for preparation, it's important to remember that Glyphosate is most effective when the new growth is relatively young and there is enough leaf surface area for the herbicide to make good contact with the plant.
Growing Deer together,