Keep in mind some basic things about how the equine digestive system is designed and works. Our goal should be to keep from messing it up with what we feed them. Simplified: They are designed to graze so fiber is ideal. In your case easily digested fiber with extra, healthy calories. Starch is not good (there's always some in the grazing and they are designed for dealing with those very low levels of starch, but you don't need to be adding anything that adds more than they would get from nature.
Basically grains = starch so avoid grain. Corn is the worst, but even processed oats (the most digestible) which is still 40% starch and less than 50% digestible. I won't get into the problems it does create and can create (it's been posted to death on another thread and just don't feel like digging through the medical information and tests again). Here's the link if you want to read about it. Got started over a question about feeding corn. https://www.horseforum.com/horse-nutr...d-corn-362217/
Healthy way of putting on weight:
Beet pulp is excellent. About the easiest digestible fiber available. The fiber digesting hindgut microbes love it (that's where the starches create problems). Higher nutritional value than any hay (about 30% or more depending on the hay).
Copra is excellent. Super easy to digest and is basically dealt with in the foregut.
Both very low in starch so that the hindgut isn't having to deal with more of it. Both high in healthy calories.
Using a high quality hay with is also great (forage is the ideal for a horse).
Rice brand will help with the extra calories, but if you go that route I'd work on getting them off of it when the weight is up (and don't over feed it). Most grain brands are like junk food (i.e. the horses love it, but that doesn't make it a healthy choice....think Cheetos, etc....
). Great for getting them to ingest things though (think medicine coated sugar cubes for children....if you're old enough to remember those days
Not that it has anything to do with your specific issue (but is often over looked by owners as the feed industry strives to lead people around by the nose
If you're grazing over a large, diverse grazing area you should be ok, but if you're relying mostly on a limited choice of grazing and the rest is feed and hay then you might want to check on the needed amino acids, and that the mineral levels and ratios are acceptable.
Don't lose sleep if it's not "text book" perfect (the amounts and ratios aren't an exact science) just get it acceptably close. It will change some throughout the year (e.g. mineral content in hay can vary depending on source, when it was cut and even age). And what my friend in the next county uses to balance out his horses diet might not be what I need since, even if we feed the same hay, our grazing is different (ground can have different mineral content so the vegetation will also).
How do the feral horses manage? Horses that roam over 200,000+ acres are functioning in the way that nature designed their bodies to work (both outside and inside). They will graze on a very diverse assortment of vegetation with varying mineral and nutritional value on different days. The closest thing they'll come to grain is if some of the grass has gone to seed, but will amount to a very small volume and will be very limited (some horses don't even care for it
) Very few people have the luxury of being able to keep horses under conditions which are that natural so ideally we have to try and simulate what they would get under those conditions.