Sources of energy in horse feeds

When the subject is horses, the word "energy" can have different meanings depending on the context. Misunderstandings can arise quickly when discussing feed, because although the meaning of energy is clearly defined in the field of nutrition physiology, not many riders are familiar with it in this context. However, knowing how the energy content of a feed is calculated, and the effects of different sources of energy on the organism, will help you in choosing the right type and quantity of feed for your horse. 
 

Feed in numbers: energy has a "currency"

A horse's body must maintain a healthy temperature, renew tissue, and perform such processes as muscle activity, lactation, or growth. All these processes require energy, which the body must absorb through food.  Certain nutrients can serve to generate energy after digestion in the course of metabolism. 

These include carbohydrates (starches, sugars), fats, proteins, and for horses, crude fibres (cellulose, pectins) as well. 

Manufacturers of horse feeds are required to subject their "crude nutrients" (crude protein, crude fat, crude fibre, crude ash) to laboratory analyses and include this information on their feed packaging. These crude nutrients can also be used to calculate a feed's energy content. Many manufacturers have meanwhile begun to include the calculated energy content on their feed labels. The common unit for this is the megajoule (MJ) of metabolisable energy (ME). In human foods, one finds the equivalent in the form of kilojoules and kilocalories. 
 

Hay vs. oats: How much energy does your hayrack or manger hold?

Simply put, the energy value in MJ per kg indicates how fattening a feed is. Once this system of energy evaluation is understood, it quickly becomes clear: the common wisdom about hay not being fattening is not entirely correct. Hay has an average energy content of 6 MJ/kg, just over half of the energy content of oats (10.8 MJ/kg), nevertheless a horse's energy requirements can and should be fully met with hay. (More below!)

If the horse takes in more energy than it needs through hay, over time it will gain weight – because at the end of the day, the number of MJ ME will show whether the horse takes in energy corresponding to its needs or if the energy intake is larger (or smaller) than required. The same applies to oats. Their energy content is nearly double that of hay, which makes them quite fattening for horses. We can also do away with the widely held misconception that oats just make a horse "fizzy" and are not suitable to maintain proper weight. 

When riders speak of "energy", they usually do not mean the actual energy content of a feed, but rather the contained nutrients' effects on their horse's performance. 2 kg of hay may have more energy than 1 kg of oats, but horses' bodies react less sensitively to hay than to oats when it comes to riding performance and behaviour. This is because the energy provided by these two feeds comes from different sources. When it comes to a horse's fizziness under the saddle, an energy source's effect on blood sugar level plays an important role. The "energy" that a horse displays whilst being ridden is thus not to be equated with the metabolisable energy (ME) that it takes in through its feed. 
 

Green energy: The horse as (r)evolutionary hindgut fermenter 

Observing a 600 kg horse, it is hard to believe that such a large animal can adequately nourish itself by eating grasses and herbs. This is made possible through billions of little helpers in the horse's large intestine. Whilst crude fibre is only pertinent to intestinal health in the form of dietary fibre for us humans, evolution has turned the equine hind gut into a kind of "biogas plant". Here, countless microorganisms break down plant cellulose which the horse cannot digest otherwise, and thus produce volatile fatty acids continuously that the animal can then use as an energy source. 

The entire digestive tract of the horse is perfectly adapted to this form of energy generation. Over the entire day, the horse crushes fibrous feed with its teeth so thoroughly that the fibres can be easily broken down by microorganisms later. Its stomach is relatively small due to a constant intake of small portions, and its capacity to digest fats and carbohydrates in the small intestine is limited. In normal cases, the absorption of fatty acids from the large intestine ensures that the horse's blood sugar level remains constant and that there is enough energy to maintain all vital processes. 

Even though today's horses have undergone significant visual changes over the years through breeding, their digestive tracts are no different from those of their wild ancestors. This means that the bodies of modern sport horses are perfectly adapted to use crude fibre as their primary energy source. In all horses, too little fibre structure in feed can lead to problems in the digestive system (insufficient dental abrasion, gastric ulcers, colic, etc.) and even to behavioural issues. For this reason, the following should be observed: the energy maintenance requirement should always be covered by a sufficient quantity of roughage. For a warmblood weighing 600 kg, this metabolisable energy requirement is approx. 63 MJ, corresponding to about 10.5 kg of hay, whereas a robust 400 kg horse has an energy maintenance requirement of approx. 36 MJ, corresponding to about 6 kg of hay. 
 

A general rule of thumb: 

Depending on its type, a horse needs approx. 1.5 to 2 kg of hay per 100 kg of body weight per day to meet its energy maintenance requirement.  


The energy requirement will rise with physical activity, depending on the intensity of the work. For most leisure horses, which do not necessarily work every day and usually for no more than an hour, this may be only slightly higher than the maintenance requirement. But the work requirement may be just as high as the maintenance requirement or exceed it, so that the daily energy requirement may be more than double.

The daily energy requirement for most horses lies between 25% and 50% above the maintenance requirement. Hay alone is not always sufficient to meet this energy requirement. And then there are other possible energy sources which do not primarily provide energy in the form of crude fibre.

 

Carbohydrates? Yes and no. Not all horses are the same, and neither are all
sugars.

Carbohydrates have a bad reputation when it comes to horse feeds, whereby, strictly speaking, dietary crude fibre consists of carbohydrates. Normally, when it is said that a horse's carbohydrate intake should be limited, this naturally does not involve crude fibre, but rather the carbohydrates that the horse can digest in the small intestine (e.g. sucrose and, to a certain extent, starch) or those which are broken down quickly into volatile fatty acids and lactic acids by bacteria in the large intestine (e.g. undigested starches, fructans). These carbohydrates are often simply referred to as "sugar" in horse feed. 

Horses are essentially not designed to ingest large amounts of these carbohydrates. The sugar in sugar cubes, malt beer, or molasses is just as unhealthy for horses as it is for us humans, and they should be given these only under special circumstances, if at all. These carbohydrate sources do not make suitable concentrate feeds. 

Starch from cereals, on the other hand, behaves differently. Most horses do not need cereals as concentrate feed, as they also can meet their energy requirements through cereal-free alternatives; nevertheless, starch in feeds can sometimes be justified for higher energy requirements. We take a closer look at when that is the case and what to keep in mind in our article Starch in horse feeds. Cereals (oats, barley, maize) are high in starch, as are milling by-products like brans (including rice bran!). 

For healthy horses, a rule of thumb:

Up to 1 g of starch per kilogramme of body weight per meal. Up to 2 g of starch per kilogramme of body weight per day. 


What about other carbohydrates? Fructans and other easily fermentable carbohydrates from grasses are found in all horse feeds. It's hard to find a roughage that doesn't have a total sugar content of between 10% and 15%.

And the hay that's currently available may not always deliver what the horse needs. Whilst sport horses may benefit from nutrient-rich hay with a higher sugar content, this quickly presents an oversupply for leisure horses, who will then steadily gain weight just from eating hay.

Carbohydrates also play a role in the pasture. The sugar content in grass can fluctuate greatly depending on the season, temperature, and weather conditions, and naturally this will have an effect on the energy content. Horses and ponies with low energy requirements will therefore become fat if unlimited pasture grazing is allowed. 
 

Fat digestion without gallbladder: (no) problem?

Because carbohydrates are not ideal sources of energy for every horse and then only in limited quantities, it's worth looking at vegetable oils such as linseed oil. Fat has the highest energy density of all energy sources. 100 g of vegetable oil will give a horse about the same amount of energy as 300 g of oats. At the same time, fat won't affect a horse's temperament and make it "fizzy" like oats will. This is because the digestion of fat doesn't affect blood sugar levels despite an enormously high energy intake. That's another reason why the use of oils in feeds is interesting, and not just for horses that cannot use carbohydrates as energy sources. Oils do not affect blood sugar levels so they do not "go to the horse's head" and their high energy density allows for a reduction in concentrate feed. 

Many riders nevertheless remain suspicious about adding oils to their horse's feed. This is because most horse owners know that horses don't have gall bladders, and often mistakenly presume that this means that horses cannot digest fat. This, however, is not true. A horse may not be able to store bile to make fat accessible for enzymes to digest, but it produces bile in the liver. Not having a gall bladder only means that a horse cannot digest large amounts of fat all at once. 

Whilst dogs, for example, have no problem if half of their daily food intake consists of fat, this would knock the horse's gut bacteria out of balance. The small amount of bile released in the small intestine means that fat passes into the gut undigested and creates an imbalance in the gut bacteria there. This is because fat has antibacterial properties and impedes the activity of microorganisms in the gut. Larger quantities of oil therefore should always be fed to the horse in several small meals over the course of the day.

A rule of thumb:

A healthy horse can digest up to 1 g fat per kilogramme of body weight with no problems. The fat content of each meal should not exceed 0.5 g per kilogramme of body weight. 


The calculated maximum quantity corresponds to a half litre of oil per day for a warmblood horse weighing 600 kg – with this information in mind it becomes quickly clear that the conventional quantities of between 50 and 100 ml of oil are nowhere near enough to overwhelm the digestive tract of a warmblood horse.

Nevertheless: always introduce oil to the horse's feed slowly, increasing it by only 5 to 10 ml per 100 kg per body weight per day. If you are still not sure whether your horse can tolerate oil, pay close attention to its stools: if the horse is consuming more fat than it can digest, this will cause an imbalance in gut bacteria, resulting in shapeless, soft mounds instead of well-formed balls. Most horses will stop eating feeds containing oil long before this happens. Unlike humans, horses generally do not perceive fat as being tasty. 

Cereal-free feeds based on fibre and fat are very good for meeting increased energy requirements without having a negative effect on blood sugar levels, particularly for very temperamental horses or those with metabolic disorders (like Cushing's disease or PSSM).

Oil can also be beneficial for horses with stomach problems: its high energy content allows for less cereals to be used, and it has beneficial antibacterial effects. The upper area of the stomach is home to microorganisms which convert easily fermentable carbohydrates into acids. Oil inhibits their activity and thus reduces the release of acid. 
 

Proteins: tricky energy sources

Protein is also a source of energy. However, it behaves somewhat differently in the body than crude fibre, carbohydrates, or fats. Since all body tissue consists mainly of proteins and all endogenous enzymes are also made of amino acids (the "building blocks" for proteins), protein in feed mainly serves for the building and regeneration of tissue and enzymes. Only when the amino acid requirement is met will the body also draw on proteins from feed as a source of energy. 

In metabolism, the use of protein as an energy source is then comparatively complex: the body must first break down the amino acids which can serve as energy sources. This produces toxic ammonia, which the liver must convert in further metabolic processes into non-toxic urea, which in turn is excreted by the kidneys in urine. Horses can tolerate an oversupply of two to three times their protein requirements without problems – for sport horses in particular, a certain oversupply of protein is unavoidable because they need to consume large quantities of feed to meet their high energy requirements, and feed will always contain a certain amount of protein. However, since energy production from amino acids is quite complex and much is still unknown about whether the additional strain on the liver and kidneys will create problems over time, high-protein feeds are not ideal sources of energy for horses. During certain phases – for growth, muscle development or milk production – the body needs more amino acids to build tissue and then protein-rich feeds like lucerne, linseed cake or brewer's yeast can be valuable supplements. More information on this topic can be found in our article Protein in horse feeds
 

Energy: too much adds weight, too little takes weight off

A horse must consume roughly the same amount of energy as it burns every day to maintain its weight. More on one day and less on another won't make a big difference, but intake and expenditure must be kept in balance over time. 

If the horse takes in more energy than it can burn through metabolism, heat loss or physical work, the body will store the surplus energy in the form of fat deposits. This storage is an evolutionary form of insurance against lean times. Wild horses enjoy a surplus of nourishment during the summer months. In winter, however, vegetation is sparse and less nourishing. The horses can now draw from the fat deposits from grasses eaten in the summer until food is again available in sufficient amounts in the spring. 

Today's domesticated horse does not experience hunger in winter, yet in the summer months it frequently takes in much more energy than it needs through pasture grazing. Some breeds that are easy doers (Haflingers, Fjord horses, Gypsy Cobs, but also Iberians, Barbs and Arabians) have been bred to have such low energy requirements that they can become severely overweight through consuming large amounts of hay alone. To keep that from happening it is absolutely necessary to reduce hay intake to eliminate an energy surplus (generally corresponding to 1.5 kg of hay/100 kg target weight).

 If a horse is already overweight, it will need exercise in addition to a reduction in energy intake, because physical exercise burns extra energy. If an energy deficit results from these measures, the body will begin to draw from its own reserves. Short-term (for example during training), glycogen – the stored form of carbohydrates – will be depleted from the liver and muscles. Long-term, the energy deficit will cause the body to draw on fat deposits and muscle protein as sources of energy. To prevent the loss of too much muscle mass whilst on a diet, the horse's feed should have less energy but not less protein. This is because muscles continue to burn energy when not in use, thus making it easier to lose weight. 

Slim horses should always be given adequate protein, otherwise they will become gaunt over time. If a horse with an ideal weight is given additional work, its feed must be adjusted accordingly. If hay is no longer sufficient for the horse to maintain its ideal weight, it may be given concentrate feed. Depending on individual needs, this can add more energy – through fibre, carbohydrates, and fats – to the daily feed plan. We are happy to advise horse owners in choosing the right concentrate feed for their horses. 
 

Individually tailored energy sources from our AGROBS product line:

  • Haferwiese Sportmüsli: The fibre content of this müsli lengthens feeding times and provides nourishment to gut bacteria whilst its moderate starch content from oats and valuable fats from oil-bearing seeds are sources of highly available energy. High-quality proteins support strong muscle development.
  • AlpenGrün Müsli, AlpenGrün Mash and AlpenGrün Pellet: Our complete line of cereal-free concentrate feeds are also ideal for horses that cannot tolerate starch. High-quality dried green fibres and oil-bearing seeds continually provide energy without affecting blood sugar level or temperament. 
  • Horse Alpin: Our oat-free pellets with moderate starch content and easily digestible dried green fibres. Essential fatty acids from linseeds and vital trace elements and vitamins for equine health. 
  • Maiscobs: High energy with unbeatable low protein and moderate starch content. High in crude fibre for balanced gut bacteria. 
  • OMEGA3 pur: This oil scores high with valuable omega-3 fatty acids, natural vitamin E and high energy density. It contains no starch or sugar and is suitable for all horses that can use an extra portion of energy. 

Celina Hofmann, veterinary surgeon
October 2020, © AGROBS GmbH

Sources:
-    Coenen, M.; Vervuert I.: Pferdefütterung. Georg Thieme Verlag KG, Stuttgart, 2020 
-    v. Engelhardt, W., Breves, G., Diner, M., Gäbel G.: Physiologie der Haussäugetiere. Enke Verlag, Stuttgart, 2015
-    Gesellschaft für Ernährungsphysiologie. Empfehlungen zur Energie- und Nährstoffversorgung von Pferden. DLG Verlag, 2014
-    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