In a case like this, I would strongly recommend over engineering the floor. I know how expensive farm equipment can be...
It might also be advisable not only to use thick decking, but also run two layers, the top layer run at a right angle to the bottom layer. This will help to distribute loads across the decking boards, which will in turn help to distribute the weight on the joists more efficiently. It will also lessen the impact of a failure in any single piece of decking.