I've read somewhere that horses are supposed to eat a certain % of their body weight in forage in a day. Can anyone tell me what it is? Had a google and I can't find it
Absolute minimum is 1,5% of their body weight in forage - dry weight. Ideally they should have 2,5%.
Hay is nearly 100% dry weight so you can pretty much just calculate the weight they need directly. Grass is a bit trickier since it goes anywhere from 98% (dried grass in the Summer) to 10% (Spring grass).