Sugars supply forages with energy required for re-growth and are a nutritional component
needed by both forages and horses. However, some horses, likes some humans, are sensitive to
the sugar content of hay and pasture forages, which can lead to potential health problems. These
problems include: laminitis (founder), equine metabolic syndrome, equine Cushing’s syndrome,
or forms of tying-up such as recurrent exertional rhabdomyolysis (RER), polysaccharide storage
myopathy (PSSM), equine polysaccharide storage myopathy (ESPM), and others. In general,
horses with obesity and metabolic syndrome, Cushing’s disease, and pasture-associated laminitis
are unable to take up sugar in the form of glucose into their tissues because they have developed
a diminished response to the hormone insulin (“insulin resistance”). Obese or fat horses and
horses over 15 years of age are more predisposed to insulin resistance. In contrast, horses with
PSSM have increased sensitivity to insulin and take up too much glucose into their muscle
tissues. Because some forages can contain high sugar content, whether hay or pasture, they may
be problematic for horses with these specific conditions.
Many of the forage crops available today were researched and developed for cows with calves at
their sides. Most are cool-season grasses that are commonly found in horse pastures and hay
fields around the upper Midwest and may be high in sugar. These sugars are in the form of
fructans as well as simple sugars like glucose, which posed no problem for cows. When
techniques were developed to analyze forages for “sugar”, they did not distinguish between the
proportions of fructan and simple sugar. The difference may be important for horses because
glucose is absorbed in the small intestine and triggers an insulin response, whereas fructans are
passed to the horses’ hindgut and fermented without triggering insulin. Diets high in simple
sugars and the type of starch found in grain are not good for horses with insulin resistance as
they result in very persistently high blood sugar. Horses with a predisposition to laminitis should
not be fed forages high in fructans. This is because fermentation of fructans in the large intestines
by microbes can upset the microbial balance, and makes the gut environment more acidic which
releases toxins into the bloodstream that exacerbate laminitis.
Unfortunately there are no “silver bullet” grasses that are consistently low in sugar. Most coolseason
grasses, like as orchardgrass and fescue, can have high sugar content. Timothy and
crested wheatgrass tend to be medium in sugar content, as are most warm season grasses.
However, warm season grasses are usually not winter hardy for the upper Midwest, and cannot
compete with cool season grasses in the spring and fall. This creates a dilemma for horse owners
with horses who are sensitive to high sugar content or have been diagnosed with one or more of
the above problems.
Alfalfa hay/pasture is not the solution for horses that are sensitive to sugar content. Alfalfa tends
to be higher in digestible energy, calories, and protein content than grass. More calories can
result in weight gain, and high levels of protein can lead to glucose production in the liver.
Currently, it is thought that a safe sugar content for sensitive horses is 10% or less. Testing is a
good starting point for determining sugar content in hay, but caution should be used to ensure
that a representative sample is taken. Just looking at a forage sample or knowing the species will
not give you an estimate of sugar content. As a last resort, up to 30% of the sugar content can be
flushed from most grasses by soaking in water (60 minutes in cold water and 30 minutes in hot
water). Care should be taken to ensure all soaked hay is eaten, as unconsumed damp hay will
Determining sugar levels in pastures is extremely difficult because of the many factors that must
be considered. Sugar content is highest when grass is in the vegetative state (early spring and
during re-growth); during periods of cool nights and warm sunny days (fall or early spring); after
a hard freeze; and during drought conditions. Sugar is usually stored in the top two inches of
forage growth, and even brown grass found during a drought or winter months can contain high
amounts of sugar. Until more cool-season grasses with low sugar content become available,
careful pasture management and forage testing by horse owner with sensitive horses is essential.
Good pasture management entails not overgrazing, limiting grazing time, and/or using a
Currently, a research project at the University of Minnesota is underway to better understand the
role sugars play in sensitive horses.