Problem and background of the discussion.
It has been known that horses and ponies that are fed too much carbohydrate by accidental grain overload may develop laminitis. Another important cause of laminitis is consumption of lush pasture. Recently specific carbohydrate fractions (fructans) have been proven to be associated with the onset of laminitis (van Eps and Pollit, 2006). The horse ingests fructans mainly during grazing (fresh grass) or in the roughage originating from grass. The fructan content of grains is generally low (and most concentrates will therefore have low fructan content). Horse owners are currently informed about the possible relationship between “fructans” and the occurrence of pasture associated laminitis. However, there are indications that the horse community is confused by the use of mixed terminology for carbohydrate classification and the classifications of the diseases in this discussion. In this article we try to give you some more insight in the terminology and fructan levels in pasture. Questions like “which horses are at risk for pasture-associated laminitis” and “how can horse owners reduce the risk for pasture-associated laminitis” are discussed in this article.
Terminology and fructan production in the plant
From a nutritional point of view the diet can be divided in some approximate fractions like for example Carbohydrates, Fats and Proteins. However, the (plant) Carbohydrate fraction can be further divided in several fractions. In figure 1 part of this categorisation of plant carbohydrates is presented (NRC, 2007). One can derive from figure 1 that starch, sugars and fructans are different fractions of cell contents. However, fructans can be categorized in either one of two groups (Fig 1) depending on the the type (chain length) of fructan. Fructans are normally fermented in the hindgut (Fig 2) and result in volatile fatty acid production (VFA).
Starch is the major “storage” carbohydrate in most “warm-season (C-4)” grass and legume seeds. However, many of the “cold-season (C-3)” grasses have fructans as their “storage” carbohydrate. Thus, when more energy as a result of photosynthesis is produced than needed by the plant, this energy is converted to fructan and translocated from the leaf to stem. Fructan production is not self limiting, allowing high levels of fructan to accumulate (NRC, 2007). Cold season grasses are often used in pastures in Western-Europe.
Carbohydrate fractions and there relation with laminitis
Starch & sugars are digested by enzymes in the small intestine of horses. The horse can coop with starch and sugar levels up to about 2 g/kg BW per meal((NRC, 2007). Cereals and grains (f.a. mais) generally contain high amounts of starch. However, when the amount of starch and sugar intake exceeds the capacity of the enzymes to digest all starch in the small intestines, undigested starch may flow into the hindgut. High amounts that enter the hindgut suddenly, may result in rapid fermentation (Fig 2) and increase the onset of laminitis. In practice, this may accidentally happen when the horse has escaped”from it’s barn and has gained free access to starch rich feed (large meals of concentrate will contain lots of cereals). Thus high amounts of starch may lead to laminitis (Fig 2).
The role of starch and sugar is also important in other pathological conditions like glycogenic storage diseases and insulin insensitivity. In these cases starch and sugar intake must be as low as possible.
The study of “Van Eps and Pollit” versus Fructan levels in pasture grasses
The researchers Van Eps and Pollit (2006) were able to induce laminitis in (primed) horses when fed 7.5-10 g fructooligosaccherides /kg body weight (BW). Fructooligosaccherides1 are categorized as a fructan like compound. This would imply that a 500 kg horse would need to ingest 3750-5000 grams of fructan to induce laminitis.
General plant studies (Chatterton et al, 1989 cited by NRC, 2007) observed a maximal fructan content of 450 g fructan /kg DM when 180 warm- and cool season grasses were grown under laboratory conditions. However, Gräßler and von Borstel (2005) reported an average fructan level of 50.7 g/ kg DM measured in 10 mixed grasses and legume horse pastures in Germany. The highest fructan levels in this study were found in a breed of Lolium Perenne (Perenial Ryegrass) and Lolium Multiflorum (Italian Ryegrass). The highest fructan level observed was 140.2 g/ kg DM (fourth cut). The NRC (2007) further cited a study from Volaire and Lelieve (1997) that observed fructan levels in orchardgrass in the Mediterranean that accumulated up to 400 g fructan/kg DM in the stem bases. In fertilized rygrass species, fructan contents of 75-279 g/kg have been observed in the United Kingdom. More studies have been executed but it is clear that the fructan levels in pasture grasses can vary between 50 and 450 g/ kg BW. In the study of Van Eps and Pollit it was stated that horses can consume up to 15 kg pasture dry matter (DM) daily. Thus, in practice a fructan intake that may reach similar levels of horses as used in the study of Van Eps and Pollit may occur when the horse is grazing all day on a high fructan pasture. However, in the pre-mentioned study, the researchers induced laminitis with inulin and not with fresh grass. It is unclear if the horse would ingest such high fructan levels that are mainly present in the stem when the horse is able to explore its normal grazing behaviour.
Important factors that explain the observed variation in pasture fructan content are: genus and species of the forage, light intensity and duration, temperature, nutrient and water status, state of growth and grazing management. Environmental conditions that enhance photosynthesis but reduce growth (e.g. a high light intensity coupled with cool temperatures) allow elevated levels of non-structural carbohydrate levels (Fig. 1). In Spring or Autumn these weather conditions may typically appear. Under these conditions pasture fructan levels may rise and these conditions may be linked to the onset of laminitis for horses that are at risk for these conditions.
Horses that are “at risk” when pasture fructan levels are high
There are specific groups of horses that may be more sensitive to develop-pasture induced laminitis. Horse breeds that are more susceptible to “equine metabolic syndrome (EMS)” may be at risk for pasture induced laminitis when grazing on pastures with high fructan levels. Currently the term “EMS” has been adopted to describe a collection of clinical signs that contribute to the development laminitis in horses. This syndrome is currently defined by the presence of insulin resistance2, obesity and/or regional adiposity and prior or current laminitis (Frank, 2007). For clarity, horses that have laminitis do not need to have EMS! Horse and pony owners that describe their horse as an “easy keeper” may be more susceptible to EMS. Easy keepers may be more adapted to grazing on poorer forages and readily gain weight when out on pasture.
Horses that are obese or insulin resistant may also have an increased the risk for laminitis when they have free access to pastures with a high fructan content. Obesity increases the risk for various disease conditions and should be fed a restricted diet. Horses with for mentioned conditions (insulin resistance, EMS) have problems dealing with non-structural-carbohydrates in general (Fig 1).
Practical pasture management
Horse owner can take the following considerations into account to prevent pasture associated laminitis.
- Gradually adapt your horse to pasture.
- Choose grass species that accumulate relatively low amounts of fructan (not Perenial or Italian Ryegrass); It is unlikely that horses grazing on typical “horse” pastures with low quality grasses will ingest high amounts of fructan. Horses grazing (non-fertilized) pastures that were used for dairy cow grazing often contain grass breeds aimed to have a high WSC-content. Horses on these pastures may be at a higher risk.
- As indicated, various environmental conditions determine the fructan content of grass. Especially, horses at risk for laminitis should get restricted access to pasture with high fructan accumulating grass breeds during periods with a high light intensity coupled with cool temperature (Spring and Autumn) or draught.
- Draught and low levels of fertilisation are conditions that favour fructan accumulation.
- It is difficult to predict the pasture fructan content as it is influenced by many factors and may therefore vary. For example, if the previous day has been sunny and the night has been cool, fructan levels will be high in the night and morning. The best time to give predisposed horses access to pasture is the late afternoon -early evening. However, if the previous day has been sunny and the night has been warm, fructan levels will be low during the night and morning and rise in the afternoon especially when there has been draught/ insufficient fertilization. The morning would than be a suitable period for grazing. Turnout in the paddock may be an option for predisposed horses during critical periods.
- Roughage may also contain fructan levels (mainly in the stems). It is advised to soak forage for 1 hour for predisposed horses.
- Appropriate Pasture Nitrogen fertilization management; Increasing the growth of the plant during spring increases growth and may prevent energy to be converted to fructan.
- Fructooligosaccherids are also used as prebiotic in equines. The daily dose of these products is much lower. The reader should not confound this type of research with the effects of “overload” studies that are for example done by van Eps and Pollit (2006).
- Insulin resistance: a decrease in tissue responses to circulating insulin, which causes a decrease in insulin-mediated glucose uptake into skeletal muscle, adipose, and liver tissues (Kronfeld 2005, cited by Frank, 2007)
- NRC, 2007, Nutrient Requirements of horses. The National Academies Press
- Gräßler and von Borstel, 2005. Fructan content in pasture grasses. Proceedings Equine NUtrition COnference, 1st – 2nd October 2005, Hannover, Germany
- Frank, N. 2007. Diagnosis and management of insulin resistance and Equine Metabolic Syndrome (EMS) in horses. In: Apllied equine nutrition and training, Equine Nutrition Conference 2007. Edited by Arno Lindner.
- Van Eps and Pollit. 2006. Equine Laminitis induced with oligofructose. Equine Vet. J. 38:203 -208