The fire was reported around 5:30 a.m. The trailer caught on fire, and the tractor was able to separate from the trailer and pull away, the Iowa State Patrol said.
West bound traffic was being rerouted, troopers said. The east bound lane remained open, but smoke was a problem.
There was no immediate information on what the truck was hauling, and there were no reports of any injuries, the patrol said.
Dexter is about 30 miles west of Des Moines.