How to keep weight on a hard keeper senior horse. Senior horses' digestion becomes less efficient, and they may run out of teeth. These situations require changes in their feeding strategy. Since all horses require a forage-based feeding program, for senior horses, this may mean adding soaked hay cubes and beet pulp to the diet, to make up for the senior horse not being able to chew hay or grass well anymore. A high fat, high fiber senior grain with added vitamins and nutrients (I use Nutrena ProForce Senior), also fed soaked if teeth are bad, provides extra calories to help the hard keeper senior put on and keep on weight.