Show horse needs to gain weight

Hello,
I have a thoroughbred mare who's been a hunter show horse for me the past several years and since i've moved her from one barn to the other she's lost some weight and I'm having trouble getting her to beef back up. Could you recommend me some thing I should feed her so she gains her weight back. I need something that will guarantee her gaining the weight and I'm sure someone out here could assist me. Thank you.

What was she being fed at the other barn and what is she fed now?

She's being fed pelleted Select feed and was fed the same thing at the other barn. Three medium coffee can sized scoops twice a day plus access to round bales almost all the time.

What kind of hay is she being fed in those round bales? How much time is she spending out vs. stalled? Is she turned out with other horses or alone?
(all going to play into what is suggested)

I have a TB who lost weight while he was at my trainer's, and to beef him back up I have him on Blue Seal Charger and beet pulp pellets.

I don't know how big your TB is but mine is only 15.2 h, and he's getting four coffee cans of Charger and one of the beet pulp twice a day. He's averaging 11-12 lbs. Of feed a day.

I'm not a fan of Select because while it can maintain weight, it's not very good for weight gain.

If you can afford it, I suggest Triple Crown Complete. If I had a Southern States dealer close to me who carried it, that's what I'd be feeding.

JJ's also getting 30 lbs. Of orchard grass/timothy hay a day from square bales that were cut and baled specifically for horses.

JJ has put his weight back on slowly, which is what you want. A rapid weight gain is hard on their joints, back, legs, and feet. Plus, too fast and they can founder.

JJ's out most of the time with my two Arabians, but I stall him when I feed everybody. The Arabs would blow up like balloons and die from founder within a week if I let them anywhere near JJ's feed. The both of them together don't get as much as JJ does!