Probiotics will help with digestion as well as stress. I use FastTrack Equine Probiotics, it is cheaper than the Probiocs name brand. About $19.95 per 5 lbs. I have a barn of 4 minis and 13 ponies - I give one TBL per day, evening feeding (doesn't matter, that's just what I have time for). So the bag lasts over a month.
I also started feeding ground flax seed and that has helped tremendously. And I add 2 Tbls. of soy oil to the mix to make sure they have proper oil.
And I add wet beet pulp to their feed. It is weighed wet and depends on the size of the horse/pony getting it as to what they get.
Then mine have good grass hay during the day, for minis one 8 qt. pail slightly packed, which equals about 1 lb. And I use the ratio of 1.20 per 100 lbs of horse/pony. So if a pony weighs 500 they get 3 lb.s of hay per day.
In the evening, a pony would get 2.5 lbs. of alfalfa hay or hay cubes after their grain.
This balance has kept 'hay bellies' off my young ones, yet allowed them to grow as much as they are growing. I have a yearling filly who is as big and weighs as much as my 2 year old fillies. So I know they are doing fine on this method.
Fastrack I believe has made a difference overall.
I just wanted to add, that I feed per their weight at the time of feeding, not to their potential weight. I have heard that going around and have seen horses and ponies get fat or have hay bellies.