I had all four of my horses go off the vit/min supplement I was feeding them --- one at a time over a period of a couple weeks.
The Expiration label says the feed is good until October, 2013, plus I keep that very expensive stuff in the spare bedroom to keep it from going bad. This stuff costs $60/bag and it's got me so honked off that I am sending a sample out for analysis to see if there's something "brewing" in that bag that shouldn't be
There seems to be no rhyme or reason as to why they all stopped eating this product and, yes, they all pound their hay down, like your horse
I have changed vitamin/mineral supplements and they are once again eating.
Since weight is an issue for your horse, replace the beet pulp with pelleted equine rice bran and see how that works. Rice bran is 22% fat and provides the horse with cool energy.
TSC sells Manna Pro's Max-E-Glo. It is specially formulated for horses and is stabilized so it won't get rancid. It comes in meal form but I prefer the pellets.
If you're already adding water to whatever goes in the feed pan, try a 50-50 mix of 100% pure apple juice (I use WalMart's brand) and see if that helps.
Don't buy the apple juices that have added sugars in them, WalMart's brand does say "100% pure" without anything added. I drink a lot of bottled water and will just drink one half way down, then fill it the rest of the way with apple juice, as needed.
As far as getting his vit/mins in him, you may end up having to buy something on line that only requires 1 - 2 ounces be fed,daily. Eight quarts is a LOT of feed to shove down him and he may simply be sick of it and by now he's sick of the taste of everything
If possible, I might try feeding him three times a day but, I know that's about impossible if the horse is boarded and/or you work outside the home.
Hope this helps