Pasture-associated laminitis (PAL) is a form of insulin-induced laminitis that can occur after horses have grazed on grasses and legumes that are high in hydrolyzable carbohydrates (HC).

This condition causes pain and stretching and weakening of the laminae of the hooves. If it is not treated promptly, it can progress to founder – a life-threatening condition that involves displacement of the coffin bone in the hoof capsule.

In a survey of American horse farms, over 50% of reported laminitis cases were thought to be due to grazing on lush pasture and grain overload. [4]

Lush grass contains higher levels of NSC – a measure of the sugar, starch, and fructan content of forage. Transitioning horses onto nutrient-rich grasses in the springtime can trigger a laminitic attack caused by the insulin response to high sugar and/or starch intake. [1][2]

Horses that have metabolic conditions involving insulin resistance, like equine metabolic syndrome are at risk for pasture-associated laminitis. [3]

Pasture Associated Laminitis

A common cause of lameness, pasture laminitis can occur in any breed or age of horses. It can affect any of the four feet, but most often occurs in the front hooves.

Pasture-associated laminitis is most likely to occur in the spring and early summer when new growths of grass have low fibre and high sugar content. It can also occur in the fall when many pastures experience a regrowth and cooler nighttime temperatures cause plants to retain higher levels of NSCs.

All pasture plants contain non-structural carbohydrates, but concentrations are higher in periods of rapid growth, with exposure to direct sunlight, and after times of stress such as drought or frost.

Pasture laminitis involves impaired blood flow to the laminae of the hooves – soft structures that anchor the coffin bone to the hoof wall. The condition can cause separation of the laminae and lead to a loss of support to the coffin bone. [15]

When the coffin bone is no longer supported by the laminae, it changes orientation within the hoof capsule and may rotate downwards causing pressure on or perforation through the sole. If laminitis progresses to this state, the condition is referred to as founder.

 

Healthy vs. Laminitic Horse Hoof | Mad Barn CanadaIllustration:

NSC Content of Grasses

Pasture growth contains fluctuating levels of simple sugars, fructans, and starch.

There is no single type of pasture grass that consistently has low NSC values because these levels are affected by a number of dynamic factors, including:

  • Type of plant
  • Soil composition
  • Use of fertilizer
  • Duration and intensity of sunlight
  • Time of day
  • Season and cutting
  • Exposure to environmental stressors
  • Height of grass

Cool season grass species including ryegrass, timothy, fescue, and orchard are known to be high in fructans and sugars.

Warm season grass species including Bermuda, Bahai, and crabgrass do not produce fructans but may be higher in sugars and starch. [13][14]

NSC concentration in grass typically increases during the morning, reaches its highest levels in the afternoon, and then declines overnight. Horses consume less NSCs when they graze in the nighttime or morning versus the afternoon.

Stress conditions that restrict growth within plants also increase the concentration of NSCs. When growth is restricted, energy produced by photosynthesis is stored as a reserve rather than being used for growth.

Stressors to plants include low temperatures, killing frosts, applications of herbicides, overgrazing and poor soil composition.

Causes of Pasture Laminitis

While the clinical signs of pasture laminitis are observed in the hooves, this condition begins in the digestive tract after carbohydrate-rich plants are consumed.

Pasture-associated laminitis is referred to as a form of endocrinopathic laminitis because it involves elevated levels of insulin – an endocrine hormone.

The exact mechanisms that contribute to this disease are still under investigation, but it appears to be mediated by elevated insulin levels (hyperinsulinemia) in the horse’s body. [15][17]

Although alterations in hindgut function and fructan overload have previously been implicated in PAL, new research demonstrates that elevated insulin is the main causal factor. [18]

Insulin Resistance

Pastures comprised of nutrient-rich grasses and new growth will supply high levels of easily digestible carbohydrates, including sugar and starch.

Sugar and starch are rapidly digested energy sources that increase blood sugar (glucose) levels and contribute to more insulin being secreted by the pancreas.

Horses that have metabolic syndrome or