Access to public land owned, managed, or funded by the state of Missouri for horse and mule use shall not be denied on trails and roads that are currently designated by the state as land upon which horses or mules can be ridden, except that access can be denied where conditions are not suitable because of public safety concerns, necessary maintenance, or for reasons related to the mission of the agency that owns or manages the land so long as a written statement is posted at the trailhead stating the cause and estimated duration of the closure. Nothing in this section shall cause horses or mules to be excluded from inclusion in the development of new trails on Missouri public lands.