Is there a tarp law in South Dakota?

No, a tarp is not required, however the vehicle must be constructed or loaded as to prevent the load from dropping, leaking or sifting or otherwise escaping. You don’t have to tarp it, but nothing can fall off.

The full statute can be found at SDCL 32-15-18

We often get complaints when a driver gets a windshield chip from leaking gravel, rocks, or other objects. A trooper is unable to write a ticket for this unless the trooper witnesses something falling off the load. If this happens to you, if you have enough details such as license plate numbers of the vehicle, time, location, and other details, the county State’s Attorney may be able to assist you. Often the vehicle’s company is able to assist as well so you can try to contact them.