I had some weight issues with Daisy with all the moving she did. Poor girl lost close to 200 lbs. Thank God she needed to lose it because she was in the obese category when I got her. I want to keep her where she is at, she's perfect, but with our workout schedule she needs extra calories and with no coat to speak of she burns them like wildfire. So after talking with my vet we put her on 6 flakes a day, 4 lbs of Allegra Cadence, that may increase toward spring when she get harder workouts, and 6 Cups of rice bran. We chose the Cadence in this case because you don't have to feed bucket fulls of the stuff as you do with a senior food to get the same results and it's just as digestible. It was split at two feeds but now we're going to do three. Even though she's only 9 years old, I would have gone with a senior feed if she was idle. Every horse is different and even those who are non working have widely varied metabolisms. This can give you a ball park idea of what I need to do just to keep her weight on, without trying to put any on her, however, she is a very large horse, in medium work conditions and needs this amount to keep her fit.
Daisy cannot tolerate alfalfa so grass is our only option but any more than 6 flakes and she just makes it a bed and frankly our grass hay here is a bit too expensive for bedding
: The more often they are fed in smaller amounts the better IMO. It's more what their systems are built for.
I suggest whatever you decide to feed run it by your vet and get some blood drawn and a general checkup too, especially if this is a recent happening. Always feed by weight not volume and if you don't have one get a weight tape to track his progress so you can note earlier if what you are doing is working vs using the "eye" method only. Most feed stores carry them and mine only cost a few bucks.
Hope your boy is getting plump soon. They love to make us worry don't they
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ETA I noticed you said he was floated but if it was done with hand floats it may be like nothing was done at all. Apparently the hand floats cannot adequately do parts of the mouth due to it's configuration according to the dentist. I found that even though Daisy has been floated regularly, she was still full of hooks especially toward the back and it was well worth the money to have a pro equine dentist do her with power tools.