I feed nothing but hay to every one of mine (youngest ever being about 3 1/2 months, though he is about 18 months now, to well up into their 20s), they get no supplements beyond a salt block and loose minerals. They all stay fat, healthy, and happy year around.
One of the most serious problems that people have in the US, though many don't realize it's a problem, is severely overfeeding their horses. So many people think that "Oh, just hay can't possibly be enough for them, they need this or that or this other thing too." What most folks don't realize is that horses evolved eating grasses, hays, twigs, even the bark of some trees. They simply aren't designed for many of the grains and feeds that they are given these days, then people wonder why their horses are so prone to founder or colic.
We stopped feeding grain of any kind in the mid 90's, when my Dad stopped showing. Since then, we've never had a single case of colic or founder with any of the hundreds of horses that have gone through our barn since then. Every one of the horse's gets either grass or alfalfa hay and nothing else.