Horses are at increased risk for laminitis when grass begins to grow in the spring, providing abundant forage that is high in moisture and carbohydrates but low in fiber. Autumn is also a high-risk time for grazing horses for a somewhat different reason.
Although pasture growth doesn’t look as appetizing with its browning color signaling the beginning of dormancy, this seasonal change triggers plants to produce and store carbohydrates. Waning daylight hours, cool night temperatures, overgrazing, mowing or several dry days followed by rainfall can all lead to increased carbohydrate intake as horses graze. This overabundance of nonstructural carbohydrate (NSC) disrupts the chemistry of the horse’s digestive tract, leading to the release of endotoxins that can result in the onset of laminitis.
A second contributing factor is the rise in adrenocorticotropic hormone, or ACTH, that occurs in all horses as days get shorter and air temperatures drop. This natural change in hormone level might not be a problem for horses with normal metabolism, but for those with a condition such as Cushing’s disease or equine metabolic syndrome, higher ACTH levels can trigger increased insulin production. This rise translates into more risk for laminitis in susceptible equines.
She recommends that horse owners should eliminate or limit fall grazing for insulin-resistant horses or those that have shown signs of laminitis in previous years. Offer low-NSC hay to provide fiber, soaking the hay if necessary to remove some of the water-soluble sugars. When the grass reaches full dormancy and the lower stems are no longer green, it’s probably safe to allow horses back into the pasture, at least until forage growth picks up in the spring.