Gradually build time and intensity as the horse's fitness improves. Some horses are natural pasture potatoes. Ride, longe, drive or work the horse on a treadmill rather than rely on free choice exercise. Provide plenty of clean, fresh water so the horse's digestive and other systems function as efficiently as possible and rid the body of metabolic and other wastes. Select feeds that provide plenty of high quality fiber but are low in total energy.
Measure feeds by weight rather than volume to determine appropriate rations.
Select feeds that are lower in fat since fat is an energy-dense nutrient source Switch or reduce the amount of alfalfa hay fed. Replace with a mature grass or oat hay to reduce caloric intake. This will also satisfy the horse's need to chew, reduce boredom, and provide fill for its stomach. Feed separate from other horses so the overweight horse doesn't have a chance to eat his portion and his neighbor's too. In extreme cases of obesity, caloric intake may also need to be controlled by limiting pasture intake.