I am not familiar with your tractor but I have a car that does not have a drain plug. You have to pull the lower hose to drain it.
I guess anything to cut cost.
Here is a jpeg of the manual on draining the cooling system.
No matter how you do it, following the manual will result in a real mess.
Here is what I think is a better solution. After removing the radiator cap, push a small, 3/8" hose down into the tube. Either use a pump or siphon the the coolant out. You may have to let it set a little while during the process to allow the coolant to drain to the bottom of the system. If you need to get 100% out you will most likley need to remove the hose as shown in the manual.
The siphon thing works better if you raise the tractor up on some blocks to get a better drain action.