A interesting thing about horse grazing is that horses unlike cattle are "continuous grazers", they do not stop to ruminate like cattle. Studies have shown that horses will eat on the average 13 different feeding/grazing episodes a day totalling 18 hours compared to cattle 8 hours a day - the issue here is that horses will consume alot more forage then necessary for minimum body maintenance, i.e. DMI should be .02 of body weight just like cattle however horses will eat a good half to double that amount per day, especially a Gypsie Vanner light draft type. This is why horses on lush pasture often blow right up and get real heavy and alot of laminitis cases are from spring green up when too much sugar is present in forage.
Therefore, we can feed/graze using two strategies, low quality add lib hay, say in the 8% CP hay allowing the horse to eat more volume then necesary or feed with a higher CP and DE hay and meter out the more exact amounts of hay to meet the .02 DMI. However, based on the history of digestive and laminitic upsets that horses often die from it is obvious that horses have evolved to subsist on low quality, high intake diets inorder to keep things working ok, so I always suggest to my clients that the low quality, more high fiberous diets fed more add lib are more healthy and usually economically feasable because you often pay more for the calories then the fiber.
Lets' clear up the terms, Laminitis is the case Hay Wilson describes, Founder is the symptom of laminitis when the structure of the hoof wall sluffs off enough and the anchoring of the extensior process ligiment on the anterior hoof wall gives way and the coffin bone begins decending and rotates downward. Not all laminitis cases involve founder.