I think it should be possible to feed the horses the grass-entity, which already are possible to harvest with shears, also giving it a useful function other then decoration.
I personally think it'd be pretty cool for horses to eat the actual tall grass, not unlike how sheep are able to consume grass from dirt. It'd be just one more immersion thing for me -- going out every so often to bone meal up the field for my herd of horses to be able to graze. But that's just me, I like farming and stuff.
grass-entity? do you mean the Grass block? Tall Grass? I don't what you mean. and also there's no grass-entity. The wiki is open 24/7 that might help you
Feeding horses the grass-entity