My first thoughts:
1. 8 quarts of feed is an awful lot UNLESS he is being schooled for at least an hour every day.
1.1 If that is the case, he would benefit greatly from receiving all that feed three times a day instead of two. It could be that he just does not want that much food at one time.
2. Ulcers are also a possiblity, regardless of age; again another reason to feed three times a day.
3. Whose senior feed? It could be the senior feed isn't helping and all he needs is the beet pulp (stabilized/calcium fortified equine rice bran works just as well), and a good vit/min supplement.
I have two mid-20's Fellas that are now hard keepers. Both have ulcers. I feed both of them three times a day. No grain. They both eat rice bran and a vit/min supplement.
The horse with hind gut ulcers can't eat much of anything so I top dress his rice bran with a liquid
vitamin/mineral supplement. It only takes one ounce. He is has gained some weight and is sassier than he's been in a good long while.
These two also get alfalfa. The Arab has recently decided he doesn't want Standlees' tim/alfalfa cubes, all nice and mushed up. He needs those amino acids and protein so I mix about a pound of Standlees alfalfa pellets in with his morning rice bran and ulcer meds - he eats that - heh heh
Both these guys have serious weight issues and both have ulcers. Even with the hind gut ulcer guy's $90/mo for Succeed, I don't spend anywhere near $150/mo to feed both of them.
It can take some experimenting and creativity. My most creative thought was when I came up with the liquid vitamin idea. Liquid 747 Tuttle's (Supplements - Vitamin Mineral)
It only takes one ounce daily, and look at the cost of a gallon. The shipping is what gets me
If my local tack/supply store isn't willing to order Liquid 747 in, I am going to buy a case of it to help reduce shipping cost.
This isn't a whim for me. Along with hind gut ulcers, my 25 yr old TWH has equine metabolic syndrome and suspicioned lipomas in the GI tract. He cannot each much of anything anymore. His life is on the line and this stuff is working for him in ways I would never have imagined --- so far
I hope this made some sense but the bottom line is I think the horse is getting too much of good things and something just isn't working.
This may be a case of "less is more", therefore it might be time to strip back and start from scratch