My TB who needed to put on weight we fed her: Spillers cool mix, just grass, dengie, sugarbeet and soya oil. She put the weight on well thank goodness and the amount of food she had she didn't get fizzy. I didn't ride her for 2 weeks and then got straight back on and she was fine.
I feed it to my 28 year old as he struggles to maintain his weight. Because you feed it like a mash he finds it easy to eat. I also fed it to my 36 year old as he had no teeth so couldn't eat anything else!!!