help old horse gain weight

I have a 26 year old standardbred, Maggie she is pastured with two lively and fat quarter horses on limited turnout. She became depressed over the winter, she was at home by herself and lost a lot of weight even though she was on free choice hay, 24 hour turnout and pretty much all the grain she could want. Right now she is a bcs scale 3.5 or so. She is on 4 pounds of nutrena life design senior, and we just added 1/2 pound of nutrenta 10 10 sweet feed, but are building her up to 2-3 pounds, and about 1/2 cup of corn oil. The problem with her is that she sometimes won't eat. If we try to feed her in the morning, more grain she won't eat. What we are feeding her now is pretty much the max she will eat. Her and her pasturemates are out for 4 hours in the morning and 3 hours at night. We also put Maggie out at night with her grain hopeing she will eat more grass. She has no worms: fecal egg count followed up with deworming and retest. And her teeth were checked by an equine dentist and very good condition. What do I do to put more weight on her?

First I would drop the sweet feed, Then add 1-2lbs of stabilized rice bran and about 7-10lbs of alfalfa pellets per day. I also use hard keeper solution. I use triple crown senior, I'm not sure how nutrena senior compares but I've had great results with triple crown. My 36 year old gelding lost a lot of weight over winter and with this he has gained a ton of weight, he is a 14.2hh arab/quarter and I weight taped him at 850, I wish I had taped him before to have something to compare it to, but from looks he has gained a ton of weight.

This may sound rather stupid, but I also have a 26 year old mare, who has lost a bit of weight and sometimes only nibbles her hay and refuses to eat alot on somedays. What I have noticed is that if I pull her out of her stall, even if its just 10 minutes, everyday just to groom her or walk her around, or especially if I ride her it actually makes her happier and she'll eat most, if not all her food on those days.

I have a similar situation with my 22 yo QH gelding. I posted on here for help and someone told me she used about 1 cup each of beet pulp, alfalfa pellets, oats and black oil sunflower seeds all twice daily. He's been on it for about 2 days now and so far he loves it. You do have to watch out for founder though because of the super richness of it. So far seems to work ill keep posted.

She's never in her stall, either in her pasture or the paddock but I will try some of the things you posted. We can't ride her very much, but I could take her on some trails just hand walking