My favourite thing in the winter is to mix shredded (soaked) beet pulp with the soaked alfalfa. Easy, and the horses like it. But in the summer I cut out the alfalfa entirely and just mix the BP in their grain. Adds some extra water, easy to digest, and helps set up a good environment in the gut – all of this is a really big deal for my geriatric cushings pony, and he chews it really well to boot.
I don’t know that one is more effective than the other as far as weight gain goes (especially since 70% of the time I feed them together). I think it really depends on what else your horse is getting as far as grass and hay (and grain) goes.
And, of course, if any of your horses are having trouble maintaining weight, it may be time to be de-wormed or have the teeth floated 😉