Firstly, why is your horse 'going on stall rest'? What is his 'lameness' that you mention in the subject?
Yes, 20lb(or is it 30?) is a LOT of alfalfa, esp for an overweight horse. Alfalfa is a high energy feed that should be fed carefully to 'easy keepers'. Also feeding straight alfalfa, being so high in calcium & protein, among other nutrients, is likely to leave him with some serious imbalances.
A horse should have at least around 2% of it's bodyweight in forage daily, even if it needs to lose weight, so assuming 800lb is his ideal weight & you're feeding 20lb, that's probably about right, but most or all of it should be grass hay or lower energy feed if he doesn't need to gain.
Sounds like you maybe don't know much about equine feeding/digestion if you're worried about colic for that reason. I suggest you do some research & I'd also consider consulting a nutritionist regarding appropriate supplementation.
Horses are designed to eat tiny amounts, near constantly of low grade roughage. So long as you're feeding free choice or little & often, so his stomach's never empty, so long as he's getting at least 1.5-2%bwt of roughage, he's not getting high starch rations & you change/add feed gradually, colic shouldnt be a prob.
He has an ongoing issue with one of his legs, I don't wanna give out a lot of information a this point. But my vet told me to put him on stall rest as he doesn't want him running around, it's only making it worse. We already have a treatment plan set up.
I do know enough about feeding to keep my horses healthy, some stressful things happened lately and I lost track and have since caught myself. I was also already considering a grass hay just needed someone to back-up the idea.
That's good to know colic wouldn't be a problem. I've never had a horse colic before and I would like to keep it that way. His normal weight is around 750, he looks amazing when he was nothing but muscle. I'll just supply him with a good grass hay once we get him over to the new place.
Unfortunately free feeding won't be an option until September, with moving and everything I won't have the extra cash for a proper feeder to do this with. I'll just feed him on our regular schedule and make sure we make the switch to grass hay.
Thanks for your input