The following information discusses the role that carbohydrates/sugars play in equine health and nutrition. This information has been edited from a fact sheet supplied by Colorado State University Extension. The complete fact sheet can be found here.
- Carbohydrates/sugars play an important role in equine health.
- Sugar and starch are absorbed in the upper intestine and fructan has to be fermented in the lower digestive tract (hindgut).
- Feed can be tested for sugar levels. Consult a veterinarian or nutritional consultant to determine the right sugar levels for your horse.
- Feed testing facilities can provide instructions on proper sampling and recommended procedures to send samples to the lab.
- Different environmental conditions raise the level of fructan in pasture grass
Much has been written about sugar levels in feeds and forages because they play an important role in equine health. It is inappropriate to describe low-sugar feeds as “low-carbohydrate” feeds because there is no horse feed that is low in “carbs.” The fiber in forage/hay is carbohydrate in nature.
Fiber is made up of complex carbohydrates, and the horse’s digestive system needs it to function properly.
Feeds vary in sugar and starch content--the “high-sugar feeds” can be detrimental to horses that are sensitive because the starch:
- disrupts normal digestion,
- exacerbates certain medical conditions, and
- can lead to serious complications such as colic and laminitis.
Carbohydrates can be classified into three categories: simple sugars, starches and complex carbohydrates.
Simple sugars are a one-sugar unit that is readily and rapidly digested in the upper intestine.
Starches are a small group of sugar units connected together that quickly digest and absorb in the upper intestine.
Complex carbohydrates are made up of sugar units connected together that cannot be digested and absorbed from the upper intestinal tract. Those carbs must be fermented by bacteria and protozoa in the lower digestive tract or “hindgut” (cecum, dorsal colon, and ventral colon).
One important complex carbohydrate in equine feeds is fructan. It is a non-structural carbohydrate (NSC), as are simple sugars and starches, but it is different because it must be fermented in the hindgut like other “fiber-type” complex carbohydrates.
Fermentation of fructan, other complex carbohydrates, and fiber involves a longer process than sugar and starch. It also requires a delicate balance of the proper population of microorganisms in the hindgut.
Feeds will vary greatly in their sugar, starch and fructan levels. (To view multi-year averages of different carbohydrate levels in different feeds go to www.dairyone.com/Forage/FeedComp/MainLibrary.asp. Click the “OK” to the Summary Statement, then click on “Main Library.”)
Fructan is commonly found in certain types of grasses and hays, and the content varies immensely. Fructan is a type of carbohydrate that plants accumulate and store for energy. It can be found in high levels in cool-season grasses and hay made from those grasses. Cool-season grasses such as timothy, orchard grass, brome and ryegrass predominate in horse pastures and hay fields.
Normal Digestive Physiology
A horse’s digestive physiology works best when the horse eats frequently and is maintained on forage or hay. For most horses, little or no grain/concentrate is needed if they are on a good-quality forage/hay. Yet because of the physical demands humans have placed on performance horses, grains and concentrates are fed to horses.
If you talk to the horsemen of years gone by they will tell you that when their horses were working on the farm, they fed them grain throughout the day, not just in the morning and night. When the horsemen worked in the fields with horses and took a break, the horses took a break, and this meant another feeding of grain.
Here are some of the important physiological processes of the equine gut. As forage/hay is eaten, the feed is chewed and a profuse amount of saliva is produced by the salivary glands. When there is less chewing, less saliva is produced, and less saliva means less buffering of the acids in the stomach. This results in increased bacterial growth in the stomach, increased acid and the possibility of gastric ulcers.
On a roughage diet, the horse normally produces up to 10 gallons of saliva a day, which has a tremendous capacity to buffer stomach acids. If the stomach becomes more acidic than it should be (which happens on a high-grain diet or with infrequent feeding), gas is produced by bacterial fermentation. The horse is incapable of belching, so the result can be pain, colic or in severe situations stomach wall rupture.
Normally, the ingested feed doesn’t stay very long in the stomach. It starts emptying food 15 minutes after a meal. Once the food reaches the small intestine, the transit time is slightly longer. It takes a half-hour to 1½ hours to make it to the cecum, which is the start of the hindgut.
Feed can take up to three days to travel through the hindgut. It is important for ingested feed to spend the majority of the time in the hindgut. If ingesta (feed) moves through faster than normal, proper digestion and fermentation does not happen and there are serious consequences, the most serious being laminitis and colic.
Differences Between Digestion and Fermentation
There are huge differences in function between the small intestine and the hindgut. The majority of the digestion and absorption of sugar, starch, protein and fats takes place in the small intestine. In the small intestine, absorption of sugars occurs through enzymatic action. In the hindgut, fermentation brings about breakdown of the fiber into digestive derivatives called volatile fatty acids that the body can use for metabolism. This process within the hindgut gives the horse the capability to digest the complex carbohydrates and fiber found in grass and hay.
But the microflora in the hindgut responsible for fermentation are sensitive to changes in diet. An overload of high-sugar type feeds or high levels of fructan can upset the normal population of bacteria and protozoa, changing the environment within the hindgut that causes a cascade of problems.
Detrimental Effects of Feeding High-Sugar/High-Starch Feeds
Typically, grain and concentrate rations are higher in sugar and starch than forages. When horses are on a ration or diet that has a high sugar and starch content, it can have dramatic effects on the digestive physiology such as:
- There is less chewing, less saliva produced, more acidic contents in the stomach, and possibly more gas or development of gastric ulcers.
- The ingested feed moves faster through the small intestine; more sugar/starch substrate enters the hindgut upon which abnormal bacteria feed and proliferate.
- The bacteria that feed on sugar and starch grow rapidly, causing normal gut bacteria to die and bringing about inflammatory changes in the gut. The dying bacteria give off endotoxins that are absorbed into the bloodstream. This can potentially lead to laminitis. These changes can result in abdominal pain, inflammatory disease and digestive upset. Thus colic is another possible consequence.
- The sugar and starch is absorbed rapidly from the small intestine, resulting in an elevated blood glucose level and a subsequent increase in insulin in the blood.
Therefore, three serious problems that occur from ingestion of high-sugar/high-starch feeds are: laminitis, colic and insulin resistance.
The body normally produces insulin from the pancreas, and it is secreted in response to high blood sugar (which occurs after eating and absorbing sugar and starch). Insulin is used to help the cells use the sugar that is circulating in the blood. It helps transport sugar from the blood into the cells where it is used.
In a condition called insulin resistance, the affected horse has a high blood sugar level from sugar and starch digestion. A resulting high insulin level occurs and persists because the body’s cells are not responding to the insulin. Therefore, the cells are not able to transport the sugar from the blood into the cells. It could be likened in some respects to Type II diabetes in humans. The insulin is there, but there is a lack of response to it.
The body’s resistance to insulin can also be involved in laminitis in that the resultant high levels of insulin may cause a decreased blood flow to the hoof and/or the high insulin may affect the uptake of blood sugar into the laminae of the foot. The resulting effect of either is a cascading set of events that result in laminitis or a painful breakdown in the bond between the hoof and the underlying bone.
Other medical conditions that are very sensitive to sugar and starch levels in the feed are:
- Gastric ulcers
- Cushing’s disease
- Equine metabolic syndrome
- Osteochondritis dissecans (OCD)
- “Tying-up” syndromes such as polysaccharide storage myopathy (PSSM) and exertional rhabdomyolysis (ER)
Detrimental Effects of Feeding High Fructan Forage or Hay
As mentioned above, fructan is a complex carbohydrate and is not like simple sugars or starch. Typically forages or hay are higher in fructan than a grain-based feed. A feed can be low in sugar and/or starch, but if it has a large amount of fructan, problems occur in the hindgut. Remember that most of the fructan is fermented in the hindgut.
When excessive fructan arrives at the cecum and large colon (hindgut), the bacterial flora is overwhelmed, the normal gut bacteria can die, increased levels of the digestive derivatives from fructan called volatile fatty acids are formed and this can cause digestive upset and colic.
When abrupt changes like this occur in the bacterial flora, the results can be local inflammation in the hindgut, endotoxins from the dying bacteria in the gut are released, and laminitis can be one of the outcomes.
Best Horse Feeding Practices
It is best to feed horses only small amounts of high sugar/starch feeds (grains and concentrates) in frequent feedings; this will provide for optimal equine digestion and fewer medical problems. Unfortunately this schedule might not be convenient to many horse owners. It might be hard to divide grain feedings into three to four feedings per day, but that is what is best for the horse’s digestive system.
Insulin resistance is treated and prevented mainly by nutritional changes and exercise. The insulin resistant horse should not be on any grain or high-sugar concentrate feed, and they might need to be taken off pasture grass completely and only fed hay that is low in sugars and starch. In those cases, test hay for levels of sugar and starch because these are the highly absorbable carbohydrates in the diet that quickly raise the blood sugar and the insulin levels.
A proper hay would be a prairie hay or a second- or third-cutting of alfalfa hay processed after the bloom stage. But you cannot tell the sugar content by type of hay or a particular cutting of hay. It should be tested. Even the new Nutritional Requirements of Horses published by the National Research Council (NRC) does not give us a definitive value. Current thought is that a safe sugar content for sensitive horses is 10% or less.
Feeds can be classified according to their “glycemic index,” which is an indicator of how a particular feed will elevate the blood sugar level and the subsequent blood insulin level after consuming it. The elevated blood sugar and resulting insulin is called the glycemic response. Horses with insulin resistance should be on a low glycemic index feed because they have an abnormal glycemic response after eating feeds that are high in sugar and/or starch.
In a recent study the glycemic index values were determined for 10 feeds. A high index value predicts a high blood sugar response to this feed. They are listed from highest to lower:
- Sweet feed
- Jockey oats
In contrast, the following were shown to have lower glycemic index values (a low index value predicts a low blood sugar response to this feed). They are ranked from mid-range to lowest:
- Wheat bran
- Beet pulp
- Rice bran
- Soybean hulls
Fructan will be at a higher level in the plant at different times of the season and at different times of the day. It will be higher in plants in these situations:
- after a very sunny day--photosynthesis produces a greater quantity
- after a period of stress whether it is nutritional or from drought
- after cool evening temperatures--below 40 degrees Fahrenheit
- after quick and rapid drying of cut grass and hay
Good Grazing Management
Good grazing management will lower the consumption of fructan and reduce the incidence of colic and laminitis. Good practices should include:
- Limit grazing on high fructan grasses to less than one hour a day for susceptible horses.
- Use grazing muzzles when necessary.
- Don’t overgraze pastures; fructan is higher in the lower few inches of the plant.
- Fructan is lower on fertilized fields; the plant is less stressed so there is less fructan.
- Graze early morning; fructan is higher in afternoon/evening after a sunny day.
- After a sunny day and a cool night, limit grazing in the morning because the plant has not used up its energy store of fructan during the night.