Preventing "Grass Founder"
(Pasture Associated Laminitis)
Laminitis, or founder, is a painful and potentially devastating foot condition that can affect any member of the horse family (horses, ponies, donkeys, and mules). There are many different conditions or situations that can cause laminitis or increase the potential for it to occur. Probably the single most important in grazing horses is unrestricted access to lush pasture. In a recent nation-wide survey, access to lush pasture was felt to be responsible for almost 50% of all cases of laminitis. In most parts of the country, the risk for pasture-associated laminitis, or "grass founder," is highest in the spring and early summer, when plant growth is greatest.
The reason lush pasture is such a laminitis risk is because it is high in soluble carbohydrates—simple sugars and starches that are readily broken down by the bacteria in the horse's large intestine. One of the consequences of rapid breakdown of these carbohydrates is production of a substance that, when absorbed into the bloodstream, can damage an important structure in the hoof: the basement membrane. This structure essentially forms the "glue" that attaches the hoof wall to the pedal bone, or coffin bone (the bone at the base of the limb that is encased by the hoof). Breakdown of the bond between the hoof wall and the pedal bone is the basic process that triggers the destructive chain of events associated with laminitis.
Of the soluble carbohydrates found in grass, one of the most important is fructan. Studies have shown that fructan levels in the pasture are highest in the spring and summer months. On sunny days, fructan levels gradually rise during the morning, peaking around noon. They then gradually decline and are lowest just before dawn. So, the riskiest time for a laminitis-prone horse to be on pasture is between late morning and late afternoon, in the spring or early summer.
It is worth mentioning that spring/early summer is not the only time when grass founder occurs. Although far less common, it can happen during a mild, wet autumn or after a drought; in other words, any time rainfall, sunlight, and daytime temperatures are sufficient to stimulate rapid plant growth.
The good news is that preventing grass founder is simple: limit the horse's access to lush pasture. In overweight or cresty-necked horses and ponies, and in those that have had grass founder before, it may be best to keep the horse off lush pasture entirely until the grass is more mature. The horse can then be gradually re-introduced onto pasture. In the meantime, keep the horse in a dry lot and feed good quality grass hay.
Other options for limiting pasture intake include restricting the horse's pasture time to only a few hours per day (if possible, avoiding those high-risk hours between late morning and late afternoon), using a grazing muzzle, and fencing off part of the pasture to make a small paddock. (A grazing muzzle is a strap-on webbing or leather muzzle that allows the horse to eat some grass, but not a lot. The horse can drink with the muzzle on without any difficulty.)
In summary, preventing grass founder is a simple matter of keeping an eye on your pasture throughout the year and limiting your horse's access or intake when the grass is lush.
This article was adapted from Preventing Laminitis in Horses—a practical guide to decreasing the risk of laminitis (founder) in your horse by Drs. Richard Mansmann and Christine King.
Gary E. Hanes, D.V.M.
Briarwood Equine Clinic
2995 Woodside Rd. Suite 400
Woodside, CA 94062