What do you plan to use that field for when you don't have meat chickens? Will you want a pretty lawn turf, will it go back to nature, maybe a veggie garden? That would influence my choices.
But in general for chickens, think in terms of what grows well in your climate. I'm partial to native grasses, whatever yours are. Clover is a great forage crop. There are two basic types of clover, red and white, and many different varieties of each. Alfalfa is a great forage crop.
There are several garden crops they would love such as cabbage, cauliflower, broccoli, collard greens, mustard greens, chard, and kale, but I don't think those would work well with a tractor. Mabe plant those separately in a garden and feed them the leaves.
My suggestion is to chat with your county extension office. They should know your local growing conditions and what grows well. They should know your native forage grasses. They could probably recommend a specific clover. They may even have a specific mix if they are into poultry.