Join Date: Oct 2009
Location: Just south of sanity
Smudger, the person who told you this apparently knows very little about hay.
Hay isn't just a 'filler' to keep horses happy. In fact, some horses get only hay and no grain, and are perfectly healthy.
Hay has a lot of nutritional value, but like grass, it will depend on the type of pasture, what's been seeded to grow, the time of year, and other factors for just how nutritious it is at any given time.
First, second, and third (and sometimes fourth) cuttings don't usually have the same nutritional value. Depending on the cut, some will be higher in starches than sugars, and others will be higher in sugars.
Fescue for example, should not be fed to pregnant mares in their last 3 months, because there's a fungus that grows on fescue that can damage the unborn foal. The mother could abort, or the foal born with physical deformities.
Hay isn't a one size fits all type of forage. You really have to study it to know what's in your hay and what time of year it was cut, in order to know whether or not it's the only feed necessary for your horses. Some will need supplemental grain rations, in order to make up for poorer quality hay.