The DEF in the tractor can freeze it doesn't hurt anything and it was designed to allow for freezing. In freezing temps how it's supposed to work is the tractor runs w/o def for a little bit while the engine warms up, then the coolant heats up the def and then it's pumped into the exhaust where it does it's thing to clean up the exhaust. Then when you shut down, the def in the system is pumped back into the def tank on the tractor for safe storage.

However, I've heard that like last winter when it was terribly cold, the engine couldn't heat up the def fast enough and the tractor would think it was out of def and derate.  I think there's been software updates to fix that, that it would allow a longer warmup period to thaw the def before it derated, i think, but best to check with your dealer on this. Semi trucks with def had that same challenges last year too. Some dealers were installing fitted blankets over the def tanks to help hold the heat in so the def would melt. I don't know beyond that on the truck side.