What kind of hay do you feed? If you feed any kind of grass hay, your horse is probably deficient in Calcium (Ca) and Magnesium )Mg). They can be slightly deficient or very deficient with severe consequences, particularly for PG mares and growing horses.
The ratio of Ca:P is very important -- more so than the amounts of each. It should be 2:1 C:P. Calcium can even be 4:1 or 6:1 and there won't be any problems. But, if it ever gets equal or higher in P than Ca, it is a disaster.
If you feed alfalfa, you probably do not need a mineral supplement as it has more(Ca) than (P). Some alfalfa is 10:1 Ca:P and it is oftentimes blamed for enterliths (stones) in a horse's gut. Almost all horses with stone are on an alfalfa diet.
Since you live in a state that is a major cattle producing state, you can find any kind of mineral you want. We are the same here in Oklahoma. We feed nothing but grass hay and most of it is pretty mature and coarse. It is all deficient in Ca and in Mg so we use a mineral that has 4 X as much Ca in it as P. It is labeled as an 'Unmedicated Wheat Pasture mineral. It is 21% Ca; 5% P; 2% Mg; 23% salt. It also contains 150,000 IU of Vitamin A and other trace minerals and Vitamins.
Signs of a deficiency in Ca include eating wood and trees, eating and licking dirt, chewing manes and tails, foals born with weak or crooked legs and other serious problems.
Obviously, the ideal would be to have hay tested. We used to have a lot of hay tested but then came the drought and we had to feed whatever we could get. During the last two years, we have probably fed 30 different kind of hay from that many different growers and several different states. But, every grass hay we have had tested has been low in Ca and some has been very high in P. Grain products are also high in P and low in Ca. Wheat bran and rice bran are Very high in P and have almost no Ca, so they can mess up the balance when only a small amount is fed.
The Vitamin A in our mineral is probably as important as the Ca and Mg.