Notwithstanding the provisions of Section 21715, a motor truck with an unladen weight of not less than 3,500 pounds and a gross weight of not less than 4,200 pounds may tow a combination of two cotton trailers operated on the highways for the exclusive purpose of transporting cotton from a farm to a cotton gin and returning the empty trailers to such farm. Extra weight or ballast may be added to meet the requirements of this section and shall be in the form of 100-pound bags of sand.