Starch in horse feeds

The feeding of starch – in the form of cereals and their by-products (especially bran) – has long been an important element in equine diets. Working horses were important capital and needed to consume much more energy than the average modern horse. At the same time, they had comparatively little time to consume feed. Cereal feeds were long considered the most important pillar of horse feeding – they can be ingested quickly, are high in energy, and less susceptible to weather conditions at harvest than roughage.

Cereals are still widely used today to feed riding horses and sport horses. At the same time, it is now widely known that horses can only tolerate limited amounts of the starch contained in cereals, which is why more and more riders are taking pains to give their horses feeds that are low in cereals or even cereal-free. What does starch from cereals actually do in the horse's body, and when is feeding starch appropriate?
 

Starch: energy reserve for plants, energy source for animals

Starch is a carbohydrate and serves as an important energy reserve for plants. Very high concentrations of starch can be found in cereal grains: oats, barley and maize, the most popular types of cereal in horse feeds, contain almost 40, 50 and 65 percent starch, respectively.

Although the organism uses glycogen instead of starch as an energy reserve, depending on the animal species, starch can serve as a very efficient energy source when ingested through feed. Dogs, for example, also developed a considerable capacity for digesting starch as they adapted to life as human companions during the course of evolution – a capacity largely missing in wolves, from which they descended. Cats, as pure carnivores, have little use for starch. Humans and pigs, by contrast, are even able to pre-digest some starch through salivation whilst chewing.

But what about horses? As hindgut fermenters, they are somewhat special when it comes to digesting starch: horses basically are capable of digesting this nutrient, but there are a few things to consider when deciding if a horse should have starch in its feed, along with the source and quantity.
 

A journey through the digestive tract: what happens to a trough full of oats? 

If one compares the consumption of hay with that of concentrate feed, it is clear that horses are much better suited for digesting roughage than they are for digesting cereals.

For one, horses consume cereals much faster than they do roughage, meaning that a kilogramme of cereals requires less chewing activity than would the same quantity of hay. Less chewing activity in turn means that the cereal feed is not as thoroughly salivated, which results in less buffering from gastric acid after it is swallowed. The enzyme amylase, which assists in the predigestion of starches in the saliva of humans and pigs, is not present in equine saliva. Grain is also literally heavier in a horse's stomach than hay: When a horse consumes large quantities of hay, part of it is directed through the stomach's pyloric canal into the small intestine to avoid overfilling the stomach. With grain, the time required to pass through the stomach is longer and the chyme contains about one third more solid particles which thicken the stomach contents. For this reason alone the horse should not be given too-large feedings of cereals.

Another reason for limiting cereals is the presence of natural gut bacteria in the upper area of the stomach. These micro-organisms break down part of the cereals into volatile fatty acids and lactic acid. If produced in excess, these acids can damage the upper part of the stomach, as it lacks a mucus layer that protects it from acids. To minimise this risk, horses should not ingest more than 1 gramme of starch per kilogramme of body weight per feeding, and not more than twice that amount per day. In practice, however, a much smaller amount is usually sufficient to provide a horse with adequate energy. 

Digestion of cereal starch begins in earnest in the upper part of the small intestine. The starch in cereals is contained in starch granules. Their molecular structures differ depending on the type of grain. These structural differences mean that the body will not be able to break down starch from every grain equally well. Oats are known to have the most easily-digestible starch. Within the recommended maximum amounts, 80–95% of the starch contained in oats can be digested before reaching the horse's large intestine.

Maize and barley in untreated form react differently: here, small intestine (prececal) digestibility is only 22% for barley and 29% for maize. Digestibility can be improved slightly through mechanical processing (crushing) or greatly through thermal processing (extrusion). Barley and maize are therefore only well suited as horse feeds in hydrothermally processed flake form. Their digestibility is then comparable to that of oats. 

   

How is starch digested in the small intestine? 

Starch consists of two different molecules (amylose and amylopectin), which in turn consist of many individual glucose "building blocks". In amylose, the glucose is linked in a chain by only one type of bond, whereas in amylopectin there is a second type of bond causing the sugar chain to branch. Amylose accounts for only about a quarter of the glucose chains present in starch granules, whereas roughly three quarters of glucose is present as amylopectin.

The enzyme amylase, which is formed in the pancreas, splits the bonds of the glucose chains. This splitting results in shorter polysaccharides which are then split into single glucose molecules by enzymes in the mucosa in the small intestine. Now the mucosa in the small intestine can absorb the glucose and send it into the bloodstream. As a result, blood sugar levels rise and cause the pancreas to secrete insulin. Insulin is a messenger that "unlocks the doors" of the cells of various tissues. These can now use the glucose as an energy source, and the blood sugar drops back to the level it was before the starchy meal.
 

Starch as energy source: tasty and stimulating

Even if horses are not built to process cereal grains in large quantities, they can use moderate amounts of the starch contained in these cereals as a quick and valuable source of energy. One should therefore not disparage cereals in feeds across the board, but they should be used with care. 

With their high energy content and delicious taste, cereals are well suited as concentrate feeds, helping horses with higher energy requirements to maintain their weight and performance. Whilst one kilogramme of hay has an average energy content of 6 MJ ME, the energy content of one kilogramme of oats can be almost twice as high, depending on quality. It is therefore practical and quite economical to include cereals in the feed of a sport horse with a very high energy output. In addition, roughage alone is often not sufficient to cover the high energy requirements of mares that are in the late stages of pregnancy or nursing.

Cereals' high palatability is also advantageous for horses that are intensively trained: after a certain amount of exercise, a horse's appetite naturally diminishes – another reason why hay feeding alone is no longer sufficient. Trainers of high-performance horses (such as racehorses) say that it's not always easy getting their sport horses to keep eating. But even high-performance horses will generally eat hygienically sound cereals. 

In terms of performance, starch feeding has another advantage: the rise in blood sugar level after consumption will have an effect on the horse's temperament. Many competitive riders value cereals in feeds because they give the horse extra energy. The other side of the coin is when the horse is "feeling his oats" – but this applies not only to oats, but to any cereal that's digested easily in the small intestine, as they all cause a rapid rise in blood sugar after consumption. 

Since the rise in blood sugar corresponds to feeding time, a horse should ideally be fed cereals about two hours before that extra energy is needed. Do not feed cereals right before physical activity, however, as this will unnecessarily burden the stomach.
 

Blood sugar peak taboo: horses with metabolic disorders

Whilst sport horses may benefit from elevated blood sugar, it should be strictly avoided in other horses: those with insulin resistance (e.g. as a result of EMS or Equine Cushing's disease) should not be fed cereals because the excessive insulin release from blood sugar peaks increases the risk of laminitis. Cereals are also fattening due to their high energy content, which further reinforces insulin resistance: some fat deposits release large amounts of hormones, which lower the insulin sensitivity of the tissue. 

With regard to EMS, it is even worth considering putting horses from breeds genetically predisposed to insulin resistance on diets that are generally cereal-free. These include robust breeds such as British pony breeds, cobs, Haflingers, Icelandics and Fjord horses.

Heavy horse breeds and American Quarter Horses often show a genetic change in their muscle metabolism called PSSM (Polysaccharide Storage Myopathy). PSSM is what is known as a glycogen storage disease. The body makes and stores glycogen primarily in the muscles when it has absorbed more carbohydrates than it needs to cover immediate energy requirements. The energy is temporarily stored in the muscle to be available for physical activity.

A horse with PSSM can store glycogen in the muscle but then cannot use it. After a certain level, the excessive accumulation of glycogen can cause damage to the muscle. This causes the horse a great deal of pain and is manifested as equine exertional rhabdomyolysis. Because the body converts especially easy-to-digest carbohydrates like starches into glycogen, horses with PSSM should not be fed cereals of any kind.
 

Conclusion: whilst some horses benefit from being fed cereals, most horses don't need them

A majority of horses perform only light work on a daily basis, if any (e.g. ridden 30 minutes at walk, 20 minutes at trot, 10 minutes at canter). In many cases, cereal feeds are unnecessary if sufficient roughage is provided (at least 1.5 - 2 kg hay per 100 kg ideal weight; also ad libitum access for horses with high energy requirements).

Since the feeding of cereals does involve risks, it should only be done when there is justification for it. The type of cereal selected should be one that can be optimally utilised by the horse. In unprocessed form, this would be only oats, but hydrothermally processed barley and maize are also suitable. No more than 1 g of starch per kg of body weight per feeding should be provided, corresponding to a maximum quantity of 1.5 kg of oats for a warmblood horse weighing 600 kg and not more than twice that amount per day. As a general rule, cereals should be spread out over many small meals; wait at least 6 hours between larger cereal meals. This reduces stress on the horse's sensitive gastrointestinal tract. It is recommended that chopped roughage be added to the cereal feed to slow down feed consumption and stimulate saliva production. If these general rules for feeding starch are observed, cereals – which taste good, supply energy and help maintain optimum weight – can be a valuable part of your horse's diet.
 

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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