The first and most important question to help you figure out what is going on is, exactly how much (by weight) is 4 scoops of Equine Senior? Then you need to look at the amount you are feeding by weight and compare it to the recommended amount on the bag for the weight that your horse should be. If you are already feeding the recommended amount or more, then you should consider adding a bit more fat to the diet. Fat is very easily digestible and provides more than twice as much digestible energy as protein without adding to the risk of laminitis in these seniors who are more likely to have metabolic issues like insulin resistance or Cushings. You can add a cup of fat in the form of vegetable oil daily and greatly increase the amount of energy that the body has for putting on weight. But you must be meeting all of your horse's nutrient needs before you count on additional fat to really help overall.
Actually, Purina Equine Senior is a substitute for hay. It provides all of the fiber that a horse needs. That doesn't mean that you shouldn't feed hay but it would redundant to add beet pulp or hay cubes both of which won't provide long stem fiber and keep a horse occupied for hours like hay will. Beet pulp and certain hay cubes will also throw off the nutrient balance of the diet. And hay cubes require good teeth to chew effectively so in a senior horse they can increase the risk of choke and they don't do nearly as much because without adequate chewing they aren't digested as well. If anything, supplying access to fresh grass or a grass hay would be the best option just to keep your horse content.
My old man is 34 this spring and he's been living on Equine Senior with hay or grass just to keep him occupied and 1 cup of vegetable oil a day for the past 4 years. These are also the recommendations that my old boss (equine vet board certified in surgery) gave her clients with senior horses. We saw wonderful results in our patients.
Licensed Veterinary Technician