Often more than one factor is involved. Competition from younger, more aggressive horses for feed is a common cause of weight loss problems. If the horse is eating more slowly than normal, dropping wads of hay out of its mouth, or dropping grain as it eats, then it probably has a significant dental problem. Other causes of weight loss in the older horse may require a physical examination, and possibly blood work and a fecal examination by your veterinarian. This is a treatable disease that can be detected with blood work.
Blood work can also pick up signs of kidney or liver disease.
There are many senior diets on the market, and they all vary greatly in their nutrient composition. This can be very significant. For example, a horse with liver disease should not be on a high fat diet. A horse with kidney disease would not do well on a diet high in calcium and protein. All older horses, whether thin or not, should have the following in late summer.