There is not much difference in the vitamin, mineral and protein requirements between trail horses and other performance horses. The biggest differences would be the amount of calories needed and the form in which the calories are consumed. Forages (grass or hay) provide an ideal source of energy that is constantly available as the horse goes down the trail. Forages are also necessary for a properly functioning digestive tract and act as a major stimulus to the thirst response for water intake. All of these are important for the trail horse.  If the trail horse can get enough calories from forage alone, then the ideal diet would be forage with a vitamin/mineral supplement and salt to supply the other nutrients that are low or missing in the forage. Only if the horse cannot maintain his weight on forage alone is it necessary to add some kind of more calorie-dense feed.