You can also try running the tractor with the radiator cap just a 1/4 turn loose so that pressure does not build up. Run it for a short while to warm it all up making sure it doesn't overheat. Shut it down and let it cool overnight. Start it up next AM or so, and see it the problem go away.
If it does, then it's a leak somewhere in the engine read head gasket most likely. Reason is with the cap on, the water pressure in the cooling system overcomes the cylinder compression pressure and allows coolant to enter the cylinder. When it starts up it blows out the water, white smoke, for a few seconds. You may also see a few bubbles in the radiator with the cap off, sometimes hard to see, and sometimes very obvious, That's why the cap off procedure works pretty well sa there is nothing forcing the coolant in. The coolant coming in might only be 1/4 teaspoon full so checking water level is a slow procedure. Outside engine cooling leaks are usually very obvious as there is a puddle somewhere, even if you have to put a piece of cardboard down to find it, and be careful water runs along anything in the way during the drip
and ends up on the cardboard a foot away!