'big head/bran disease' in horses?

You should be working with an equine vet on this. If you don't have access to one in your locale, call the nearest equine clinic and they will help you. This is not something you should treat based on what some online stranger suggests. Anecdotal remedies are a bad joke, and not something to actually rely on when dealing with an important issue such as this. The chemistry of calcium/phosphorus imbalance requires advanced education to understand, which means you need real qualified medical professionals to advise you.