If a horse can graze, on a good pasture with an abundance of varied plant life, he will not likely be even selinium deficient. Those weeds and seeds all have varying minerals in them . If the horse has a very monoculture hay as his ONLY source of grass, then he won't be as balanced as a horse that can graze and forage in a field.
Most of the horses at "our" place never get grain, or any "ration balancer". Which is kind of add, because how do you know what you need to "balance" a ration unless you send in sample to be tested.
There are many companies and universities that study soils and their outputs (grains, hays, grass, crops, foodstuff) because it has huge impacts on the human foods and the animal feeds produced in this country, which impacts milk production, meat production (depending on the methods used ) growth rates, health, etc…
This is how we know which areas of the country are deficient in what things. For example, most of the soil in the northeast is well known for being selenium deficient, thereby making the grasses, browse, or weeds grown on it and the hays produced on it also deficient.
Hay/forage content is studied to bits in agriculture and animal sciences (plus water) and suffice it to say we (livestock circles) do indeed know a lot about what is in all the different species of grasses and hays, depending in which part of the country they are grown, and how to engineer feeds and supplements to meet the needs (or deficiencies) of any type of domestic animal.
Of course, there is a lot of hype too about product “x” being the magical cure all for whatever ails a horse, so diligence is needed on the part of the horse or animal owner to sort through it all and figure out what their particular animal needs.
Did you know that if you took eggs from a group of free range hens in your area and compared them to the eggs from the same breed of free range chickens in my area there would be marked and measurable nutritional differences in their eggs?
Nothing the naked eye could detect except perhaps the color of the yolk, but measurable differences that can effect those consuming the eggs.