Farm tractors when using the highways in traveling from one field or farm to another, or to or from places of delivery or repair, or when participating in activities or events permitted under subsection 13^* of section 304.170 are exempt from the provisions of the law relating to registration and display of number plates, but shall comply with all the other provisions hereof. The state highways and transportation commission shall have the power and authority to prescribe the type of road upon which such tractors may be used and may exclude the use of such tractors or the use of trucks of any particular weight from the use of certain designated roads or types of roads, by the posting of signs along or upon such roads or any part thereof.