In real life experience, I was almost always the only boarder that cared to know what and how much my horse was eating. My fellow boarders just didn’t care, they boarded because they wanted to go to the barn and ride and not have to deal with any other care items.
To that point, I’ll also say that very rarely did I have a BO who knew squat about equine nutrition. So, it was always frustrating as a HO who did want to know and did/does have some clue about equine nutrition. Always they’d want to throw more grain for a hard keeper, never more hay.
As a BO, I offer full board and partial board - owners can bring their own hay and grain if desired, or I can provide it, but I’ll only pick up Seminole feeds, I’m not driving all around town to six different stores. One or two supplements is fine, but if you want more than that, you provide it in SmartPaks or other packaged form that contains all supplements per meal in one package.
From the BO perspective, there’s multiple reasons why one might now allow boarders to dictate what their horse eats, or bring their own feed if its something the barn won’t feed, etc. Storage space is a huge reason, as well as cost, and convenience. If someone has a small feed room, they don’t have the ability to store 15 different grain types, whether or not it’s costing them any more money (i.e. horse owner brings it). It’s also much less convenient, even if you have the storage space, to have to pull grain from 15 different cans at each feeding, instead of from 2-3 bins. These are things that I am currently willing to put up with, because I limit my farm to eight horses max. If I had a 20+ or 30+ stall facility, there’s no way. I’d offer 3-5 feed options (i.e. a standard 12% pellet, a senior feed, a ration balancer, a growth feed), and if you don’t like any of them, you’d be free to board elsewhere.
I understand, though, being a boarder, and having a horse that needs something the barn won’t provide. If the barn won’t provide it and won’t let you provide it, sounds like that barn just isn’t a good fit for you, time to look elsewhere (or not move there in the first place). No single barn can accommodate every single possible horse. It doesn’t make them a bad BO, it just makes it not a good fit.