Have you thought about asking the barn owners if your rate would change if you bought the feed for your horse versus them buying it? If you are not satisfied with your horse's current feeding regimen, then you should change it. I don't foresee a problem with them lowering your board if you buy the horse's feed, instead of them. That way you can feed your horse what you want, instead of whatever they want. I have personally had success with Omolene 200, you only have to feed a minimum of 0.35 lb per 100 lb of body weight, so for a 1,000 horse, it would get 3.5 lbs a day. It has all of the vitamins and minerals that the horse needs, so you would be able to stop buying all of the additional supplements.
It's just a thought for you to consider though.