A trot requires about twice the energy level of the walk. Pasture to provide energy - Good source if of good quality. Poor pasture or poor hay is common cause of pot bellies on yearlings. Problems with spring grass, fescue, clover, perennial ryegrass, pasture during drought. Maintenance Energy Needs of Horses.
Good pasture can be excellent source of protein of high quality for broodmares and foals.
Most horses do not need additional vitamins other than those found in their diet. Vitamins A, C, D, E, K and the B complex are needed. Vitamins A and E are not produced in the body. Must be in the feed. The horse can make Vitamins D, C, and Niacin.