I DO have a background in nutrition and have worked as a nutritional consultant for feed companies and have been called in by Veterinarians to help farms with problems like a high number of foals born with weak and/or crooked legs, OCD problems and with horses exhibiting Pica.
Absolute fact -- Calcium is lacking in ALL GRAINS while Phosphorus is high.
Fact -- Calcium is lacking in every grass hay sample and every grass hay profile that I have seen in textbooks and IRL. I have sent in hundreds of forage samples over many years and the grass hays only varied in the amount of the lack of Calcium in them. All had an inverse ratio with more Phosphorus than Calcium. Legumes on the other hand vary a lot, but in general, have a lot more Ca in them than grass hay.
Excessive Calcium is not a problem unless it is HUGE. Excessive Phosphorus is a HUGE problem even if the excess is small. It is the reason I have taken bran out of the feed formulas I have worked out for breeding and boarding farms. It is just SO high in P that you have to add large amounts of Ca to balance it out.
I have found that most Veterinarians are not very good nutritionists. They have to study equine nutrition on their own as the Universities to not do a very good job of it during Vet School. Some of the feed companies are now getting a better Ca:P ratio in their feeds, but most still do not balance out separate grain formulas for horses eating legumes (high in Calcium) and grass hays (low in Calcium). None compensate for forages that are very low in Ca.
With that much difference in forages, obviously one grain and one mineral mixture would not be suitable for all horses. It is up to the owner to be informed enough to know if their horse needs Calcium or a 'balanced' mineral that reflects the horse needs of a 1.5:1 total Ca:P minimal balance. Horses try to tell us by eating unnatural and unpalatable things like tree bark and wood.
With mature horses, you will see few if any problems when the Ca:P ratio of 4:1. A ratio of 2:1 is what I consider ideal. I have set up management and feeding programs around that goal number for about 40 years now. There have never been any problems from this ratio with breeding stock, growing young horses and performance horses. Older sedentary horses can exist with a huge overload of P and a serious lack of Ca, but even they will start eating trees and wood when it gets bad enough.
We have helped clear up many problems other than Pica and they include reducing the incidence of OCD, contracted tendons, 'knobby' ankles in young growing horses, crooked or weak legs in foals, and many other problems.
The few wood eater that did not stop eating wood after a diet change had other clinical problems. Most were stalled and had ulcers, too little turnout, were cribbers, or had other stall vices. It is hard to say which came first. The cause and effect thing on ulcers, wood chewing and cribbing is not clear in all cases.
But, one thing is very clear to me. When horses are not confined to a stall and they eat trees and wood, they have a nutritional imbalance.
I do not ever force feed a mineral, but leave loose mineral out free choice. Magnesium, Zinc and Vitamin A are all necessary for a horse to utilize the Calcium and Phosphorus in their feed properly.
I DO force feed salt when it turns very hot or very cold. This is to prevent a horse from drinking too little water and impacting, particularly when the weather turns very hot or very cold. This is the best colic preventative I know of.