In general, keeping your horse in the pasture is much better than in a stall. Horses that are turned out have a lower incidence of colic, anxiety, fear, and less behavioral abnormalities. However, all horses cannot be out on pasture all the time and some cannot be on pasture at all. Horses with insulin resistance or those that are overweight should not be on pasture at all without a grazing muzzle because most pastures contain large amounts of nonstructural carbohydrates (NSCs) and fructan is a major part of the NSCs. Kentucky Equine Research indicates that ingesting fructans is the main reason horses develop laminitis and founder on grass; it happens because the fructans are not broken down in the upper GI tract but pass to the lower GI tract and ferment, which leads to the problem.
The amount of fructan in the grass varies with the type of grass, time of day, season, and the part of the plant ingested. It is important to know that fructans are high in periods of rapid growth of grass that usually occurs in the early spring. Fructan levels are also higher in the stems than in the leaves, so avoid overgrazing to prevent horses eating close to the ground. In the fall as the grass starts to turn brown, many folks feel it is okay to turn these insulin-resistant horses out because the grass appears dead. However, stressed grass due to drought can also contain high levels of fructans so you still cannot turn these horses out without a grazing muzzle. Also, fructans are produced in the leaves due to the photosynthesis that occurs in the daytime, so grazing in the early morning will limit amount of fructans consumed.