I have been down this road a couple of times now with older horses who are unable to chew their hay. OTTB are more challenging because some are nervous and that makes keeping weight on more difficult. Thoroughbreds do better if they can graze over a longer period of time. This settles down their nervousness and helps keep the weight on. If they have no teeth that can be difficult.
If you’re in the Midwest there is a great product called Envision by Progressive feeds. It is designed for keeping weight on race horses without making them hot. It is mainly flax seed and no carbs.
Additionally I don’t feed hay cubes – I find these are too hard for the horse to chew and take too long to soak. My preference is hay pellets. Specifically quarter inch alfalfa pellets and oat hay pellets soaked for only a couple of minutes makes them easy to chew and you don’t have to worry about the unchewed ones being eaten and expanding in their digestive system causing issues. Soaking is required for these older horses. Plus the moisture helps keep the colic away. This is the main feed for my older ones and with some added grain such as Envision, the weight has stayed on nicely without issue.
One 14 hand horse is on three large scoops of mixed pellets soaked daily and one large scoop of grain. This is a good mash in which to be able to add your supplements. This mixture has kept the weight on nicely.
If you can’t get Envision – try adding Nutrena Safe Choice Senior to the daily feed. Safe choice is carb free and designed for older horses to be able to eat knowing older horses need the calories without the carbs.
Stay away from oatmo as it can give them too much energy – which isn’t good for the older guys in the mud or on the ice.
This is what I have used successfully for more than one senior horse over the last 15 years without the teeth to eat hay. We have rescued various OTTB horses, all of whom needed more weight. The Envision is awesome but you can’t get it everywhere unfortunately.
Hope that helps you. It’s helped me.