The nutritional value of hay and other feeds sometimes changes and we are not aware of the changes. If the quality of hay or pasture is less than what the horse is accustomed to, or the nutritional value of the feed has changed, this may explain why he is losing weight. You may need to consider feeding an extruded feed that may be more efficiently digested or adding corn oil to boost the energy level of your aged horse's diet. Both are ways to keep or add weight on older horses.
The link below is to an article on the American Association of Equine Practitioners Web site on feeding the geriatric horse. The article contains some very good information and suggestions for feeding for weight gain.
