When its cold a valve opens and runs engine coolant through a DEF heater. You get a short period of time to run without the DEF flowing, then if there was no DEF the engine would derate. Tractors large enough to use DEF today also have powershifts that will not let you shift untill the oil is warmed up. By the time the oil warms enough to get full transmission shifts the engine will have been warmed up and the DEF will have been flowing for some time.