I work as a consultant for farmers in Tennessee and find myself in Burley fields every now and again. For insurance purposes, the farmer is probably not going to let you out there while they're working in the field, lest you get caught up under a tractor wheel.
As for those who dark fire their 'backer, best of luck in visiting the barn when it's in operation. The wood species and cuts used as well as the process by which they use them is generally proprietary and the farmers don't even share that info with each other. Best you can get is if you live near a dark fire house, you can smell the air and try to determine which wood you're smelling. I do know at least one farmer uses slabs of hickory, oak, and sycamore because I was lucky enough to see the slabs after they were delivered.