Grains, nowadays often hidden in muesli or pellets, have long been an important part of horses' diets. When horses used to work in the fields, they expended much more energy than our current sport and leisure horses. They often worked hard all day and had few breaks to replenish their reserves. They had to eat while the farmworker had his sandwich. Grains can be eaten quickly, contain a lot of energy, and are also easy to transport and do not spoil as quickly. Thus, grains became an important component of the diets of many working horses. We now know that the digestive system of horses can only process limited amounts of starch – which is abundant in grains. Yet, many people still feed their leisure and sport horses concentrated feed made from grains. Out of habit, or because they are afraid that their horse will not have enough energy to grow or work, for example.

Starch as an energy source

Starch is a carbohydrate, plants often store their energy reserves as starch. Oats, barley, and corn contain 40, 50, and 65 percent starch, respectively. These grains are commonly used in horse feed. Humans and pigs can digest starch well, and dogs can also derive some of their energy from grains. To release the energy, starch from grains is converted into glycogen in the digestive tract, which then serves as an energy source in the blood. However, digestion works differently in horses. Horses digest their food in the hindgut, mainly in the large intestine. This is different from humans or dogs. Horses have a large amount of gut flora, which converts fibers into sugars and fatty acids that are easy to absorb.

Starch digestion in horses

Horses are designed to eat roughage with a high fiber content, with chewing and saliva being an important first step in digestion. The enzyme amylase is involved in starch digestion in humans and dogs, but horse saliva does not contain amylase. Additionally, a horse quickly devours a kilo of grains, while it takes much longer to eat a kilo of hay. This means less chewing and less saliva. This often leads to problems in the stomach, as horse saliva is important in buffering stomach acid. Less saliva means less buffering capacity. Furthermore, a horse has a small stomach, which is not suitable for processing large amounts of heavy grains at once. They stay longer in the stomach, potentially causing it to become overloaded. This is why you should never give too much pellets at once.

Different types of starch

After passing through the stomach, starch granules from grains are digested in the upper part of the small intestine. The extent to which this process succeeds varies by grain type. Oats are highly digestible for horses, with 80 to 95% being absorbed before reaching the large intestine. The starch granules from corn and barley have a different molecular structure and are much less well absorbed. Digestibility is only 22% for barley and 29% for corn. When grains are processed with heat (extrusion), digestibility improves significantly, almost comparable to oats. Thanks to amylase and other enzymes in the small intestine, starch granules are converted step by step into glucose and absorbed by the intestinal wall. This way, sugar (glucose) enters the bloodstream.

Starch in the large intestine: not intended

Horses should not consume more starch than their foregut can process. Starch that reaches the large intestine undigested is not a good idea. In the large intestine, fermentation of fibers takes place, with the help of a wide range of beneficial microorganisms. Starch acts as easily digestible junk food for these bacteria and fungi. They rapidly convert it, leading to the production of volatile fatty acids and lactic acid. The lactic acid subsequently lowers the pH in the large intestine, making the environment more acidic. Many microorganisms are not equipped to handle this acidic environment, which can lead to some of the gut flora dying off. Acid-loving and lactate-producing bacteria then become more prevalent. The pH continues to drop steadily, and in the worst-case scenario, so-called 'hindgut acidosis' occurs: severe acidification of the hind part of the digestive system. This can also impair the function of the intestinal lining, allowing harmful substances to pass into the bloodstream. These waste products, including those from the deceased gut flora, can cause significant issues in the blood. For example, laminitis. This is why it is so dangerous when a horse breaks free at night and empties the feed cart. Always limit starch intake to a maximum of 2 grams per kilogram of body weight (divided into 2 portions).

Which horses can benefit from some starch?

Modern horses no longer work long hours in the fields and perform much less labor than when they used to plow. Moderate amounts of starch can provide a quick and convenient source of energy. However, it is important to handle grains carefully. Many horses find concentrated feed very tasty and energizing. This means that horses with high energy requirements, especially those needing to maintain weight or perform tasks, can benefit from it. It is practical to feed grains to horses requiring extensive athletic performances. Some competition riders give their horses some concentrated feed two hours before the competition, so that the blood sugar peak aligns perfectly with the peak performance demanded. The horse is then more energized and spirited. However, you should never give concentrated feed or grains immediately before a performance, as it unnecessarily stresses the stomach. In addition to (elite)sport horses, mares in the final months of pregnancy or nursing a foal also do not get enough nutrition from roughage alone.

Limiting starch for horses with metabolic issues

While healthy, high-energy-consuming horses may occasionally benefit from some grains, starch is not suitable for horses with digestive or sugar metabolism issues. Horses that are insulin-resistant (such as due to EMS or PPID) should not be fed starch. It causes a spike in blood sugar levels and increases the risk of laminitis. Moreover, grains are fattening, and an overweight horse is more likely to develop insulin resistance. Horses of hardy breeds such as Fjords, Tinkers, and Haflingers often do better when fed as grain-free as possible as a precaution. Horses with PSSM1 (common in draft horses and Quarter Horses) should also avoid starch, as they have issues with glycogen storage.

Conclusion: many horses can do without starch and grains

Since most modern horses do not have to perform great athletic feats and only engage in light daily work (an hour in the arena), they do not need grains. Adequate, high-quality roughage and a vitamin and mineral supplement are sufficient for most recreational horses in the Netherlands. This also applies to horses lightly trained in dressage. Only when a horse is involved in eventing, endurance riding, or high-level show jumping or dressage, supplementation may be required. Because feeding starch and grains carries risks, it is better to feed grains only when necessary. And if you do offer grains, ensure they are easily digestible for your horse. This means: concentrated feed with oats, or extruded barley or corn, preferably spread over multiple small portions per day, preferably with some chopped roughage mixed in to ensure the horse chews longer.

In collaboration with and based on "Starch in horse feeds" by Celina Hofmann, AGROBS GmbH, October 2020 (https://www.agrobs.de/en/know-how-advice/topics/starch-in-horse-feeds-602/).

Sources:

• Coenen, M.; Vervuert I.: Pferdefütterung. Georg Thieme Verlag KG, Stuttgart, 2020

• Kamphues, J.; Coenen, M.; Eider, K.; Iben, C.; Kienzle E.; Liesegang, A.; Männer, K.; Wolf, P.; Zebeli, Q.; Zentek, J.: Supplemente zur Tierernährung: für Studium und Praxis. Schlütersche, 2014

• Geor, R.J.; Harris, P.A., Coenen, M.: Equine Applied and Clinical Nutrition: Health, Welfare and Performance. Saunders Elsevier, 2013