I just wanted to add to the above posts that what KIND of grass the horses are getting is important to consider. Even if you can't have it nutritionally anaylized, certain types of grasses are richer than others.
Pasture that is lush green and full of aflafa, clover and fescue are a founder case waiting to happen. Horse thrive on mixed grasses of variying nutritional quality. They need lots of bulk available, yes, but be sure it's the right kind. A mix is much more satisfying than a single type plant in the whole pasture. Same with hay. Timothy is a great hay,but if that's all the horse eats, there is an imbalance. So try to get mixed grass hays and just remember to watch when the grass is in a lush stage.
Also, in lush pastures, the horse not only is packing in extra rich food per bite, he's not traveling very far to get it, so a "rough" pasture often times is better than what we consider a good one. The harder they have to work to find their food, the better. It's good exercise and good for their mental health.
You should be careful about over supplementing as well. Supplements are concentrates and feeding too much of something can block their absorption of something else that even if it's in the diet, the body may not get to use it, and you know where it goes..and that's wasted money. Just something to keep in mind.