If you don't have hay you don't have hay, but that doesnt mean its whats best. Horses are grazers, their bodies are designed to constantly be nibbling on grass and hay.
MLS, we've been through this before, if you wanna weigh and measure, and make excuses for rationing fiber, and think that's best go for it. I'll never agree with you, and am not interested in arguing the point.
To the OP, if you get up in the AM and every scrap of hay is gone, you didnt give them enough. If there is alot scattered every where you gave em to much. The horses generally will only over eat the first few days till they figure out plenty is there. Shouldnt take you to long to figure out a decent balance to throw out there.