Just briefly because it is 23.38 and I have to be up tomorrow at 05.00 .....
Grace has her hay soaked for a minimum of 12 hours and more usually 24, and post soak the hay is thoroughly rinsed under running water.
A kindly soul looked into her big brown eyes, was totally hypnotised and lobbed some unsoaked hay over her door. And yes Grace did go lame and yes she also tied up. (Grace has EPSM).
Yet Grace can go out overnight into a huge field and come in just fine. Our unsoaked hay is more detrimental to her than nightime turnout. - Note she goes out at dusk and comes in at dawn.
This wouldn't necessarily be true with different hay and turnout in a different field or the same conditions but different horse. So it is important to really know your horse and their hooves. One 'size' does not fit all.