Lots and lots of hay. If your horse has no hay, then give him hay. That is what we are trying with my 22 year old off track/ex-contester/ex-jumper thoroughbred. She gets probably close to a whole bale herself over a day's time.
Also, Beet Pulp. It's cheap and it's a main component in Senior Feeds. Just Soak about a pound for a half hour and feed cold. Most horses love it, so of course mine didn't. I mix mine with molasses, brown sugar, carrots, apples, and some senior feed pellets. It's pure calories, and it will keep your horsie hydrated.
Which brings me to the next point. Make sure your horse is drinking enough water. Get him a salt block, that will encourage him to drink. Just make sure you monitor the intake so that you know if you need to offer more water depending on how much he drinks!
If none of these ways work, some Weight Supplements do work, but it's always best to try natural first!
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