It might be different if horses ingested fats and sugars and meat like humans, then sure their manure would be bad for them to stand on. But alas all they eat(or should be eating) is high fiber plant matter, so that is all that comes out the other end, sterilized plant fiber... I really do not see how that is bad for them to stand on a thin layer of it, dried of course, to help lower their sand ingestion... Even if they eat a little of it again, it hurts nothing. What do you think happens out in a pasture? Some time a horse or cow or dog or cat or rat has pooped on every inch of it, but they still have no problem eating, playing, and rolling on it.