It could have been so easy.
The question is what’s the feedstuff, wild horses eat? It’s gras. And only gras. Where would it get hay, or grain?
There are just a few problems feeding horses gras the whole year. First the availibility. Men feed hay because they need to conservate gras. And hay is the easy way. So we dry gras in leaving it in heaps to dry in the sun. We have to turn it to prevent mould and if we get really lucky there will be some rain on it. Strands break, protein, minerals and vitamins get lost. The only time a wild horse would fall back on hay would be the winter time. And we all know, how wild horses look like in the winter. They are thin. Malnourished.
So hay is just an emergency solution. Gras would be much better. There are already modern procedures to conservate gras as cobs. Low temperatures while drying (45°C in the gras, 60°C under pressure) and as short a time as possible. Of course even proceding gras as careful as possible there are changes in protein structure. But allergic horses profit from those changes. And cobs are free of mould and dust.
Then there is the problem of the composition. To feed cattle and milk cows the gras variety changed over the years. They contain more protein, more sugar and energy. And fertilization does its part, too. Contrary to cattle, our horses are mor frugal. They can’t utilize the high sugar content, they are used to varieties with high fiber and low sugar contents. Varieties that grow in higher altitudes comply with these requirements.
There’s one more fact. There are certain breeds of horses that perfect the art of utilization of feedstuff. Like shettland ponies, islandic and norwegian horses, the Haflinger, too. Horses from extrem envioments that are use too low energy food and starving. They are adapted to this feedstuff since hundreds of years. So they are more susceptible to problems like EMS, laminitis and other prosperity diseases. Age and ilnesses change the metabolism of horses, too.
It makes sence to feed a horse gras only. Only in extrem situations, like some stages of pregnancy and lactation and with race horses a problem with energy density in feedstuff may arise.