Are mud flaps required for tractor trailers in Missouri?

Yes, they are required. See Missouri Revised Statutes, Chapter 307, Vehicle Equipment Regulations, Section 307.015

307.015. 1. Trucks, semitrailers, and trailers, except utility trailers, without rear fenders, attached to a commercial motor vehicle registered for over twenty-four thousand pounds shall be equipped with mud flaps for the rear wheels when operated on the public highways of this state. If mud flaps are used, they shall be wide enough to cover the full tread width of the tire or tires being protected; shall be so installed that they extend from the underside of the vehicle body in a vertical plane behind the rear wheels to within eight inches of the ground; and shall be constructed of a rigid material or a flexible material which is of a sufficiently rigid character to provide adequate protection when the vehicle is in motion. No provisions of this section shall apply to a motor vehicle in transit and in process of delivery equipped with temporary mud flaps, to farm implements, or to any vehicle which is not required to be registered.