Two things that I am very particular about.
1. Be sure that the feed is labeled for horses. If it is intended for cattle, it could contain additives that are toxic to horses.
2. Make sure that the feed is fresh and has no mold. Mold can be toxic to horses.
I highly recommend that you have a relationship with a local equine veterinarian that can assess your individual horse's needs and help you design a feeding plan. Another advantage to having a "well visit" from your equine veterinarian is that when you have a 3:00 am emergency, he/she will be more willing to come to your place.
If no veterinarian is available, maybe there is another horse expert around.
THIS may give you some guidelines.