Basic understanding of horse nutrition, is a start, and knowing that horse feeds are divided into three main categories, not by what they only contain, but by their main content
You then can apply the KISS principle to your own horse, never be a slave to those feed companies and their marketing, and able to formulate your own diet, for each horse, based on what he needs
You can have bagged feed with 16% protein, and still not have the horse get his daily needs, if the forage is only around 9%
Nope, feed companies make zero off of me, buying any formulated bag of feed!
Sure, that sounds very nice and easy. It is, much of the time. With my other horse, I could give her any variety of forages and have her do just fine. I could leave her out on pasture at this time of the year, 24/7 with no hay and just give a vitamin/mineral supplement to round out her needs. She'd do great, and have all her needs met. I could also have her in a dry lot at night (which I currently do), and feed any of the following forages which is available to me: alfalfa grass hay mix, good quality timothy, good quality orchard grass, good quality valley grass, chaff/chopped hay either alfalfa or timothy or a mix. She'd eat enough of any of those forages to be healthy, have great digestion, enough protein, calories (energy) etc.
Unfortunately, this won't work for all horses. It's one thing to provide 24/7 forage, and another thing to have a horse actually get enough calories from it to maintain a healthy weight. For instance, if I put my hard keeper mare out on the same good quality pasture as my other mare and left them out 24/7, she would quickly drop weight down to a very low BCS while my other mare would maintain. So obviously she needs more calories than just grass can provide. Providing good pasture for 12 hours/day with hay provided in the morning for a couple hours, plus access to hay for 12 hours at night free choice also only got me a horse with a very low BCS.
So in that case, you say you wouldn't go for any formulated feed...that sounds fine too. So there are a few options, and we're still in the roughage category: beet pulp, hay pellets such as alfalfa or timothy pellets, copra, chaff. Except what happens when you offer these free choice to a horse and they won't eat more than a pound or two? So you still have a horse on a complete roughage diet, but low BCS. Yet you probably want to exercise the horse, so you need enough calories so the horse can create muscle and have enough fat for health.
If you still want to stay away from formulated feeds, you can try adding fat such as rice bran, a powdered fat such as cool calories, BOSS or some type of oil like flaxseed, canola, coconut, soy. Again that's fine, except some horses (I've discovered) don't like the taste of rice bran or powdered fats, and don't like their feed wet and oily so won't eat too much wet fat.
So now you've tried adding roughage, protein and energy in various forms, and so now what? I've learned it's not always so easy. Friends are feeding 18 hand TBs in hard work, and horses with no teeth, etc, so I don't think it's so easy to just avoid formulated feeds for everyone. I'm not trying to be rude, just facetious, but I was reading in another thread that you said if people hadn't fallen off horses they hadn't ridden green types or problem horses. I am guessing it's the same with nutrition: if you think it's simple to keep weight on horses, you perhaps haven't fed enough types especially "problem" horses. LOL.