Feed a senior horse high-energy, easily digestible foods, such as beet pulp and processed grains. Getting grain mixes with added fat or feeding the horse stabilized rice bran can also help an older horse to maintain its weight.
When feeding a senior horse a high-fat diet, owners give it higher protein and mineral-infused food as well so that it maintains a healthy diet. There are supplements for horses that have added calcium and vitamins to balance its diet. However, the horse shouldn't have any kidney or liver problems that can be worsened by these supplements.
Older horses can also become overweight or develop metabolic disorders that can be somewhat addressed with a healthy diet and medication, if necessary. For instance, a senior horse can develop insulin resistance that can be treated by feeding it high-fiber, low-starch foods to manage its blood sugar. If the horse has joint problems, give it feed containing chondroitin sulfate, yucca and hyaluronic acid to help improve joint health.
Giving the senior horse plenty of exercise and keeping its hooves properly cared for and trimmed are necessary to long-term health, as well. Exercise keeps the horse from obesity and muscle atrophy while hoof-health keeps them from injury.