Theories about why grass causes laminitis in horses range from high protein levels in young grasses to hormonal factors. The latest theory is the high sugar/carbohydrate content in some grass stages.
Growing plants have a higher concentration of soluble carbohydrates. A combination of cold nights and balmy days triggers plants to store more carbohydrates and slows their metabolism overnight so that they don’t use up as much of their stores from the day’s photosynthesis. The carbohydrates in grasses may include starch, sugars and fructans.
Fructans are complex molecules composed of fructose units (a sugar) hooked together. The plant “feeds” itself by drawing on these storage forms when it’s dark and photosynthesis can’t generate energy.
High fructan levels are also a mechanism by which grasses and other plants protect themselves from freeze damage.
In March 2002, Dr. Chris Pollitt from the University of Queensland gave a talk in the United Kingdom that elaborated on a theory first proposed in 1998 by researchers from the Institute of Grassland and Environmental Research in Wales and epidemiological studies by Dr. Karen Hinkley. That theory holds that a high level of fructan in grasses stressed by cold overnight temperatures might be why these pastures are dangerous to horses. It also states that grass-induced laminitis is similar to grain-overload laminitis.
The horse’s small intestine, which is responsible for the digestion of simple sugars and starches, has a limited capacity. If the horse consumes too much grain, which is generally 60 to 70% starch, undigested starch reaches the cecum and large intestine, which are really supposed to handle fiber.
The unusual starch overload then triggers a rapid overgrowth of the bacterial species that utilize starch. This bacterial disruption, which can also inflame the intestinal lining and make it more permeable to substances that normally wouldn’t make it into the bloodstream, is believed to trigger laminitis. Supposedly, if a large volume of fructan makes its way back to the large intestine it can trigger a cascade of events similar to that seen when a horse gorges on grain.
In his talk, Pollitt described inducing mild laminitis in experimental horses by the administration of 3 to 4 kg (6.6 to 8.8 pounds) of a commercially available pure fructan derivative, FOS, to horses. The horses also developed diarrhea and disruption in the intestinal flora similar to that seen with grain overload.
Since the presentation of these findings, many horsemen decided fructan is the cause of grass founder. However, we believe it’s too early to make this assumption.
FOS is not the same as the fructans most abundant in grass. Also, a key requirement for the intestinal disruption and subsequent laminitis that results after grain overload is the rapid fermentation of starch in the large intestine. It’s like the difference between a horse eating 20 pounds of grain at once or less than two pounds at a time over two-hour intervals throughout the day. Pollitt’s work showed that a large amount of FOS tubed into a horse could produce similar problems as grain overload, but it didn’t show that a horse grazing constantly throughout the day on grasses containing more complex types of fructan would have the same problem.
If there is one truth about laminitis, it has to be that it’s complicated. Researchers have been trying for decades to piece together the contributing factors, or identify key triggers.
So far, we have many factors identified but no solid understanding of the precise sequence of events. It may well be that while the result — laminitis — is the same, there are several different ways to get there and looking for a common pathway from such diverse causes as corticosteroids, uterine infections, insulin resistance, grain overload, black walnut toxicity, colitis and lush grass is futile. Whatever the triggers may be, the pathway is not simple.
There’s no doubt that rapidly growing and/or cold-stressed pastures cause laminitis in some horses, but why remains unclear. Fructan-induced changes in bacterial populations doesn’t fully explain how some horses can develop laminitis after grazing for only a short period. For now, we think it’s best to recognize that the high fructan content in pasture grasses may be a possible indicator that they pose a high risk to horses prone to grass founder but doesn’t necessarily boil down to the one-and-only cause or factor.