Sadly I find that there are a good many people who are afraid to feed their Minis enough. They get warned "do not feed too much" but almost never does that warning come with the second part: "but be sure to feed enough". And since the person giving the warning most often does not give any indication of how much is too much--the horses end up being given too little.
Most of my horses are in the 35" to 38" range, with the majority of those being 36" to 37". Most of them are not getting any grain, but they are out on pasture--very sparse pasture--they get a bit of grass to pick but not enough to make any real difference to their feed intake. They get hay twice a day--generally two flakes each 2x a day. My bales average 60-65 lbs and one bale feeds 10 horses one meal. So, each of these horses is getting roughly 12 lbs of hay per day. This isn't real rich hay; it is grass (timothy/crested wheat/brome) with a bit of alfalfa. It was baled in good shape but did have one rain on it prior to baling, so it's a bit dry and not "the best" in terms of nutrients. If it was really green, rich hay then they would not get quite so much. As it is, though, if I were to feed them the amount you give, they would lose weight & be much too thin. These are all mature horses.
For my yearlings --2 are 32" , 1is 34" and and 1 is 36--they are getting grain (rolled oats)...almost 2 litres each twice a day, plus all the hay they can eat--and their hay is 2nd cut alfalfa w/a fair bit of fine grass mixed in. Two year olds are 36.5" and 37" and have more tendency to get fat, so they are on grass hay (pretty much all they want to eat) and grain--and likewise, they get almost 2 litres of rolled oats twice a day. They are nicely rounded over the top line--you won't see thin horses or big bellies here. The ones I'm showing are being worked, and they're muscling up nicely and starting to be just in the condition I like to see.
If I want to put weight on (for instance, on your mare that you say is getting thin), I replace part of the ration of rolled oats with a pelleted feed. I use Frontrunner, and like their 14% pellets. My 3 year old Shetland mare is hard one to keep weight on--she is out with the Minis and needs more/better feed to stay up at her ideal weight. I bring her in once a day & give her grain--half oats, half 14% pellet--and it's working well--she is starting to round out over her topline & I'm now very pleased with her condition. A friend recently asked me about putting weight on a young QH gelding--he wasn't thin, but just didn't have the bloom the owner wanted to see. I suggested the Frontrunner 14% pellets--he's had the horse on these pellets for a few weeks now & said the horse is just right, good weight, good bloom.
