Laminitis is a debilitating systemic disease which manifests in the horse’s hoof, causing significant pain and lameness. Although there are a number of causes of the disease in horses (severe fever, grain overload, retained placenta in mares), a large proportion of laminitis cases occur in grazing horses and ponies; therefore, the term “pasture-associated laminitis” was coined. There are generally two modes in which grazing lush pasture grasses can lead to laminitis. 

The first mode occurs much in the same way of laminitis via grain overload. The high concentration of sugar in some grasses can overload the digestive system of the horse and spill into the hindgut, where microbial populations rapidly ferment the carbohydrates, producing lactic acid and causing a sharp decline in hindgut pH. Although the mechanism isn’t perfectly understood, one idea is that hindgut microbial alterations increase intestinal permeability to endotoxins, which initiate an inflammatory response and trigger laminitis.

The second mode is a slower process, often seen in chronic laminitis cases in horses with Equine Metabolic Syndrome (EMS) and insulin resistance. These horses are often overweight and have a “cresty neck.” Over time, their body loses its sensitivity to insulin, requiring more and more of the hormone to store glucose from the diet. This increase in circulating insulin also incites an inflammatory response. Understanding how the disease progresses and manifests is important when determining the best management strategy.

Cool-season grasses (such as fescue) are most often the culprit in pasture-associated laminitis cases because the sugar these grasses store during photosynthesis isn’t self-limiting. Depending on environmental conditions, non-structural carbohydrates (NSC’s), mostly in the form of fructan, can accumulate in the plant. These cool-season forages experience both diurnal and seasonal fluctuations in NSC content, with the highest concentrations occurring at sundown in the Spring and Fall. When establishing a grazing plan to reduce NSC consumption, grazing should be initiated in the early morning, when NSC content is the lowest.

With horses considered high-risk–such as those that are overweight or with EMS, insulin resistance, or Cushings–grazing fescue or other cool-season forages may need to be avoided altogether in Spring and Fall. Certain “easy-keeping” breeds also have a higher genetic predisposition to the disease. Both grazing muzzles and dry lot turnout may be options to reduce forage consumption in these horses, while still allowing for exercise. Feeding a lower calorie/mature hay before turn-out could also help prevent horses from gorging. It’s important to monitor body condition in these at-risk horses to prevent excess weight gain. 

Although encouraging pasture growth may seem counter-intuitive, proper pasture management is important to reducing NSC content in these forages. Overgrazed pastures expose the base of the plant (where most of the fructan is stored) to grazing horses. Maintaining pastures in a state of growth, with proper fertilization, and by maintaining a height of 4-6 inches is best to reduce forage NSCs. 

Well-maintained pastures can be an excellent resource, reducing feed and hay costs, providing much-needed free exercise for your horse(s), and reducing pollution from run-off. Many young horses, those that are lean and fit, and many without any previous laminitis episodes can graze safely in the spring, but remain aware of any changes in behavior or weight gain. Hooves of grazing horses can also be monitored for heat or a digital pulse, indicating a potential laminitis episode.

Horses that have been on pasture 24/7 throughout the Winter generally adapt as the pasture composition gradually changes. If you’re introducing a horse to a new pasture or if grazing has been limited through the winter, start by allowing the horse to graze for one hour then increase by 30 min every few days until a total time of four hours is reached. If you suspect your horse may be insulin-resistant, consult with your veterinarian.

Feel free to contact me at the N.C. Cooperative Extension of Polk County office at 828-894-8218 or by email with questions or should you need assistance designing a grazing plan that is best for your horse.