– Or a Necessary Source of Energy?
Many horse owners fear carbohydrates and try to avoid them as much as possible in their horses’ diets, especially sugars and starch. Carbohydrates—including sugars and starch—are, however, the horse’s primary source of energy. Horses require carbohydrates in their diet to maintain normal physiological functions, and the more a horse trains and exercises, the more energy it also needs. It is completely impossible to eliminate all sugar and starch from a horse’s diet, even if the horse eats only hay, since approximately 75% of all plant matter consists of carbohydrates. What is important to recognize is that there are significant individual differences between horses in both carbohydrate requirements and tolerance.
It is nevertheless a fact that excessive amounts can cause serious health problems, and that the physiology of the horse’s digestive system appears to set a certain limit on how much sugar and starch can be used safely. As a general rule, large amounts of sugar and starch are not recommended in a single meal. On the other hand, insufficient carbohydrate intake can negatively affect performance and stamina, and even brain and muscle function. The amount of carbohydrates a horse can tolerate—or needs to perform optimally—varies greatly between individuals. For less active leisure horses, restricting carbohydrates is often advisable to prevent obesity and digestive issues, whereas carbohydrates generally cause far fewer problems for sport horses, for whom they serve as an important energy source. If sugars and starch are not sufficiently available, the horse may be unable to perform at its best and may fatigue more easily.
Horses obtain some carbohydrates from hay and pasture grass, but whether these alone are sufficient—especially for training and competition horses—depends on the type of work expected and the level of performance required. Carbohydrates are found in forage, cereal grains, and grain by-products. Many intensively working sport horses can benefit from carbohydrates such as grains in their diet. Harness racehorses, show jumpers, dressage horses, and endurance horses performing demanding work also require energy and carbohydrates in slightly different ways.
Large Single Meals Can Cause Problems
If a horse consumes a very large amount of sugar- or starch-rich feed in a short period of time, not all of the sugar and starch may be digested in the small intestine, allowing some to pass into the hindgut. This can lead to a rapid rise in blood glucose and insulin levels. In the hindgut, microbes break down sugars, producing lactic acid that lowers intestinal pH. As acidity increases, normal gut microflora begin to die off, while acid-tolerant microbes proliferate. Some of these microbes produce toxins that can contribute to laminitis. Acidification may also cause diarrhea, gastric ulcers, colic symptoms, and damage to the intestinal lining, impairing nutrient absorption.
Different horses tolerate sugars differently, and for some, problems may arise at much lower sugar intakes than for others. In general, however, the effects of sugars are primarily related to feeding frequency, portion size, and the balance between overall energy intake and exercise level. The more frequently feed is offered in smaller portions, the better sugars are digested in the appropriate location—the small intestine. Not all carbohydrates, sugars, and starch are therefore harmful to horses, as long as intake remains moderate and appropriate for the individual’s workload.
The Type of Carbohydrate Matters
A horse’s daily diet contains two groups of carbohydrates: non-structural and structural carbohydrates. Structural carbohydrates are the fibrous components of plants. Hay provides the largest proportion of structural carbohydrates in the horse’s diet, but whole grains with hulls, such as oats, can also contribute. Horses require the assistance of billions of microorganisms in the hindgut to digest structural carbohydrates. Microbial fermentation breaks fiber down into usable energy known as volatile fatty acids.
Structural carbohydrates are essential for horses, as they help maintain a healthy digestive system and support normal gut function. They play an important role in maintaining healthy metabolic conditions, and their presence in the diet also helps regulate the intake of non-structural carbohydrates and the release of glucose into the bloodstream. This can help prevent blood sugar spikes that may lead to metabolic disorders such as insulin resistance.
Non-structural carbohydrates (including sugars and starch) are obtained primarily from grains. Digestive enzymes in the small intestine rapidly break these carbohydrates down into simpler forms, which are then absorbed through the intestinal wall. The absorbed glucose is used immediately for energy needs such as digestion, and any excess may be stored in the muscles or contribute to obesity.
Adequate intake of non-structural carbohydrates should be considered in sport horses, while moderation is especially important for horses with metabolic conditions such as equine metabolic syndrome or those prone to laminitis, including many native breeds.
As a general guideline, non-structural carbohydrates should make up approximately 10–15% of the diet, around 20% for sport horses, and less than 10% for horses with metabolic disorders (IR, EMS). Horses with Cushing’s disease also benefit from low sugar and starch intake. During the indoor feeding season, it is advisable to feed these horses forages with relatively low sugar content. For forage, the target sugar level is generally around 50–150 g per kg of dry matter, or approximately 5–15%.
Adequate Energy Is Essential for Performance
Horses require energy for different types of work: aerobic, long-duration exercise, and anaerobic, which involves faster and more intense effort such as competition. Energy can be supplied by both carbohydrates and fats.
Although fat provides approximately twice as much energy as carbohydrates, the body cannot utilize it without oxygen and aerobic metabolism. For this reason, carbohydrates—particularly sugars and starch—are immediately available as energy for anaerobic work, when fat cannot be used. Horses in heavy training generally cannot meet their energy requirements from forage alone and therefore need some carbohydrate-containing feeds. As training intensity and workload increase, so does the body’s need for sugar and starch. Intensively trained horses are more dependent on having a readily available carbohydrate source, as anaerobic energy demands are met primarily through carbohydrate metabolism. However, excessive sugar and starch intake can lead to nervous behavior and reduced performance.
Sugar and starch are also important after exercise for replenishing muscle glycogen. Muscle glycogen is the primary energy source during training aimed at increasing muscle mass. After heavy training, muscle glycogen stores can drop to nearly half of pre-exercise levels, and during endurance exercise the depletion is often even more severe. Low muscle glycogen reduces training intensity and diminishes signals in the muscle that promote growth and adaptation. Research has shown that muscle glycogen replenishment occurs more rapidly when horses receive both hay and some sugar- and starch-containing feeds after exercise, compared to hay alone.
Sugars in Pasture
A key factor in managing sugar and starch intake is knowing which feeds provide these carbohydrates. Starch is most abundant in cereal grains: maize contains approximately 70% starch, oats about 44%. Forages, including grasses and legumes, typically contain less than 3% starch on a dry matter basis. Grass contains significantly more sugar, and hay sugar content varies depending on plant growth stage, environmental conditions (such as temperature and moisture), geographical location, and plant species. Most grasses contain 7–15% sugar on a dry matter basis, but levels can rise to as high as 30% under favorable conditions, such as cool temperatures and high moisture.
It is important to remember that the amount of sugar and starch in a horse’s diet depends on how much of a particular feed is consumed. When sugar and starch intake needs to be restricted, feeds in which fat and fiber are the primary energy sources should be selected. Hay cut early in the day and dried slowly typically contains less sugar than hay cut in the afternoon and dried quickly. First-cut hay is also generally lower in sugar than second-cut hay.
Horses prone to diet-related laminitis should avoid grazing during periods when grass sugar content is high. This typically occurs in spring and autumn when temperatures are low (+5 to −5 °C), during very hot weather (above 30 °C), during dry periods, and on poorly fertilized pastures where plants are under stress. Maintaining healthy soil may also result in grass with lower sugar content.
Pasture sugar levels fluctuate throughout the day, generally being lowest in the morning after a warm night and highest on sunny afternoons when photosynthesis is most active. For this reason, at-risk horses should be turned out early in the morning and brought in by midday. Night-time grazing is also generally safer in terms of sugar content. In autumn, after cold nights, the situation may be reversed, with sugar levels highest in the morning. In midsummer, pasture grass is generally safer for horses than the lush, leafy grass of spring.
FACT:
IR refers to insulin resistance, a metabolic disorder in which the body’s cells respond less effectively to insulin, leading to elevated blood glucose levels. This can result in various health problems.
EMS is equine metabolic syndrome, characterized by obesity and abnormal fat distribution, particularly along the crest of the neck and around the tailhead.
Sources:
Al Jassim, R.A.M. et al. The Bacterial Community of the Horse Gastrointestinal Tract and Its Relation to Fermentative Acidosis, Laminitis, Colic, and Stomach Ulcers. Vet Clinics: Equine Practice, 2009.
Richards, N. et al. The effect of current grain feeding practices on hindgut starch fermentation and acidosis in the Australian racing Thoroughbred. Australian Veterinary Journal, 2006.
The Horse: Sugars and Starches in Horse Diets: They’re Not All Bad!
The Role of Macronutrients in Equine Nutrition, Haley M. Zynda, MS, Educator, Ohio State University Extension, Wayne County
Sara L. Mastellar, PhD, Equine Faculty, Ohio State University Agricultural Technical Institute
Elizabeth Share, MS, 4-H Livestock Program Specialist, The Ohio State University
