I have a general thought regarding Hard Keepers 101:
One thing forgotten about grains is that many products are meant to be COMPLETE feeds. For example, Purina Equine Senior is labeled to be fed as the horse's only source of nutrition, so you are not getting the complete complement of vitamins and minerals - and calories - that you might think you are getting if you feed hay and a few lbs of Senior per day. Obviously, good quality forage also provides nutrition, and many horses are healthy and happy getting some hay and a scoop of Senior twice a day, but it's not the way that Senior was designed to be used. Something like a ration balancer or other product that is meant to supplement forage and be complete with only a few pounds a day might be a better choice in some situations, and may be higher calorie density (to go with what DMK said). The other lesson to be gained from this is that it's not going to founder your horse to give more than a few pounds of Senior per day - it may be expensive, but, depending on how much benefit the horse is actually getting from their hay, it may be necessary to give them as much as 5 lbs at a time, 2-4 times/day. I have met horses with virtually no teeth left who were changed to JUST Equine Senior, fed according to the label or a bit more (so in the range of 15-20 lbs/day), broken into several feedings, and went from BCS 3 to BCS 5-6 once they had food they could actually chew!
What worked for us - mine is mid-twenties, hard keeper, worn down teeth so hay is an issue, and most recently getting picky and turning up his nose at his regular feeds. If there is any way your barn can feed him a 3rd meal, that helps tremendously. The Legends Omega Plus is 1800 cals a POUND. Big bang for the buck. Check calories as well as fat on whatever you try.
TC Senior as the main feed
Soaked alfalfa pellets (especially in winter, made into a mash w/TC Senior)
Legends Omega Plus
Feeding a 3rd meal/lunch
My senior is very picky and will not eat anything beet pulp based (i.e., TC senior). All winter I maintained her on BS vintage sweet, hay pellets, BS trotter (complete feed) and alfalfa pellets. She tends not to eat much hay due to teeth, but I do offer it daily. She will not touch anything that is soaked.
For awhile she would eat TNT chops, but got tired of that and really won't eat any chopped hay products at this point.
I sometimes change feeds to keep her interested. She has her own turnout next to her buddy and can snack all day, which is important.
Nutrena Life Design Senior low starch
Blue grass hay and timothy
He was on 2nd cutting Orchard grass but it gave him the runs, the blue grass/timothy works well and he cleans it up.
He is a little more ribby than I like but he's always been this way, he's extremely active so keeping weight on is a challenge. He also has cushings so gets his prascend as well and grass pasture in the summer. However, I might go with beet pulp for some extra forage. Oh, and he's a quarter horse/foxtrotter cross.
Yogurt - If you're so cultured, how come I never see you at the opera? Steven Colbert
I have a hard keeper, cushings, 16 y/o TB mare who still competes in the hunters. Finding a diet that was healthy, bet provided enough calories and fat to keep her looking like a performance horse has been a nightmare. This is the current diet she is on, although I should preface by saying all of this is subject to change if it begins to negatively affect her health:
<1lb purina senior (barn's staple feed, she's a picky eater so this makes her whole meal more palatable. I kow it's high NSC, which makes me really uncomfortable...trying to slowly cut back)
2 lbs buckeye safe n easy pellets
buckeye 25 ultimate finish
smartflex senior, herb free
1/2 cup canola oil
1 mg pergolide
4 flakes AM/PM soaked grass hay
Daily turnout on grass, with muzzle
I was feeding TC senior, which I loved. It was just too hard to get consistantly in my area, so I am trying the buckeye as a substitute.
As of right now, the mare looks pretty good. Still a little dull in the coat, but she was just recently clipped. Great weight though, looks 110% better than she did 6 months ago.
I'll echo the comment that someone made about hard keepers being just that: Hard.
I don't know anything about your boarding situation, but you should be periodically evaluating whether its the right place for your senior equine. I know that I had to move at one point because the barn I was at was very uncomfortable with the feeding routine that my old TB needed. Moving him was the best option.
That said, towards the end of his life, he got TC Senior, Soaked hay cubes (he was prone to choke), and corn oil. I also would feed Ultimate Finish which is a stabilized flax product. Ground flax is also good, but given that I'm highly allergic to it, I couldn't feed it.
Good forage is helpful, but also remember that most senior feeds CAN be fed as a horse's only source of calories and its OK. Maybe not ideal, but it can be done. When talking to my vet about my old guy and how to keep weight on him, she said that some horses when they get old just need free access to grain. Which sound horrifying, but could be the right solution in some cases, I suppose.
I was in a boarding situation last fall/winter with my hard to keep weight on 18yo Broodmare I knew the feeding had to be as simple as possible... so I started feeding pelleted Rice Bran (manna pro) in addition to Tiz Wiz Structure (similar to Safe Choice) and it helped. Also gave her all the alfalfa/orchardgrass hay she would eat.
I've moved the horses to my own (new) farm in the last month and am feeding free choice alfalfa/orchardgrass hay, Structure, rice bran, calf manna, and soaked beet pulp. She foaled last week and I wanted to make sure the baby didn't pull her down and she's actually gaining weight... I also have a free choice vitamin/mineral/protein tub avaliable for her to lick on whenever she wants and can tell a difference in body condition when she has it vs in the past when she has not been on it. My current feed mixture seems pretty technical but it's pretty easy, morning chores take me 10 minutes max.