In some posts, we have highlighted five important don’ts in horse feeding, which sparked a lot of reactions. This is likely because the feeding practices we mentioned are widely popular among horse owners.
Are these statements too general?
Anyone who knows us is aware that we generally avoid blanket statements. Especially from a therapeutic point of view, each horse must be considered and fed as an individual. For this very reason, most of our replies to messages and questions include:
“…That cannot be said in general terms, as we do not know the horse or have enough information.” or “…A thorough assessment should be carried out.”
BUT: There are certain feedstuffs where generalisations are justified, and we make them deliberately. This applies when a feed is unsuitable for ANY horse, meaning there are no exceptions. If we were to say something like “…for most horses…” instead, it would simply not be honest—it would be misleading. For the points we have highlighted, there are no exceptions.
And yes, we are well aware that there will likely be comments saying: “I’ve been doing this for years, and my horse looks fine.” We fully acknowledge that many horses are fed this way and that some may not appear unhealthy. Just because something seems to work without causing immediate issues does not mean it is healthy or beneficial in the long run.
But let’s take a closer look at this topic. In the upcoming posts, we will go into more detail on the points we have mentioned. We have already discussed haylage, brewer’s yeast, and effective microorganisms in our last article, which you can find here. Now, we continue with the topic of oil feeding:
Popular Misconceptions in Horse Nutrition – Oils
Claim: “The liver stores bile fluid.”
Bile fluid is produced by liver cells and released into the bile ducts, which begin between the Kupffer stellate cells of the liver. These bile ducts merge into a collecting duct, which in many mammals features an expansion known as the gallbladder. The gallbladder serves as a temporary storage unit for bile, similar to how the urinary bladder stores urine secreted by the kidneys.
In meal-eating animals, the gallbladder is reflexively emptied when a large, usually fat-rich meal passes from the stomach into the small intestine. The breakdown products from protein (and partially lipid) digestion in the stomach trigger the release of the intestinal hormone cholecystokinin upon entering the small intestine. This hormone causes relaxation of the sphincter muscle of the bile duct (Sphincter Oddi) and, together with acetylcholine, induces contraction of the smooth muscle of the gallbladder.
This process actively pumps bile fluid from the gallbladder into the small intestine. However, in horses, there is a unique difference: they do not have a gallbladder because evolution has not designed them for large, fat-rich meals. Instead, they are adapted to continuous intake of nutrient-poor and therefore low-fat roughage. As a result, the bile fluid continuously secreted by hepatocytes (liver cells) is released into the small intestine in a relatively steady flow.
This system aligns with the digestive process of the horse as a continuous feeder, where a reflex ensures that small amounts of food entering the stomach are gradually passed on to the small intestine. For these small, nutrient-poor portions, the steady trickle of bile fluid is sufficient to support digestion in the small intestine. The liver itself has no storage capacity for bile fluid, apart from the gallbladder in species that possess one—just as the kidneys cannot store urine.
Claim: “Bile production increases with oil feeding”
The amount of bile produced, as well as its composition, changes depending on the ingested food. This is one of the reasons why small intestine-digestible feeds should be integrated into the feeding plan gradually. The ability to adapt the amount and composition of bile is much greater in species that are specialised for small intestine digestion, such as dogs or humans, than it is in horses.
Evolutionarily, the horse is a grass eater and thus a hindgut fermenter, meaning that small intestine-digestible nutrients have always played a secondary role. As a result, horses have not needed to develop a significant adaptation to varying fat levels in their diet. The biggest difference between horses and other animals, however, lies not so much in the amount of bile produced, but in the ability of mammals with a gallbladder to concentrate bile fluid in the gallbladder before it is released, which is a function not available to horses.
In carnivores and omnivores like humans, bile fluid can be concentrated up to 5-10 times, resulting in a much greater effect per millilitre. In contrast, in horses, bile from the liver is released continuously into the small intestine, meaning there is no concentration of bile fluid. This leads to only a slight adaptation to a high-fat diet in horses. Additionally, the emptying mechanism for bile, which is triggered by cholecystokinin and acetylcholine in animals with a gallbladder, is not present in horses.
As a result, the bile fluid in horses can only be minimally adjusted to the composition of the diet. If fats are not sufficiently emulsified and thus micelles are not formed in the small intestine, fat components can be carried into the large intestine with the food mass. In the large intestine, fats can be toxic to the natural microbiome, causing dysbiosis and leading to a reduction in energy extraction from plant fibres.
Claim: “Oil is an important energy source”
Horses, due to their natural habitat in the steppe, have evolved to derive their energy needs from the fibre content of their diet. No mammal is capable of enzymatically breaking down plant fibres, so all herbivores rely on microorganisms to help access the energy stored in the complex networks of fibres. Ruminants like cows and sheep carry out this process in the stomach with the help of their rumen flora. Horses, like rhinos and rabbits, have moved this process to the large intestine, where, with the help of their gut microbiome, they extract energy from cellulose, hemicellulose, and pectins.
Of course, horses are capable of using other nutrients as energy sources, as is the case with all mammals. However, the efficiency of converting different nutrients into energy varies greatly between mammals, depending on their dietary habits. If horses are fed a diet where the energy content of roughage matches their energy expenditure through training, they neither gain nor lose weight. If energy expenditure is then increased through more training, the horse should theoretically lose weight, as it would need to rely on reserve energy. However, this does not happen immediately. Horses appear to be able to increase energy extraction from the fibre fraction of their feed through their hindgut microbiome to a certain extent, although the mechanism behind this process is not yet fully understood. Only when energy expenditure exceeds this capacity, due to further increases in training, does the horse’s body turn to other energy sources. In this case, it does not rely on stored fats, but instead on proteins. Horses can use about 25% of their body protein in emergencies for energy production.
In most animal species, energy provision during energy deficiency occurs through intramitochondrial ketogenesis in the liver, which involves the breakdown of stored body fats. These ketone bodies are released into the blood by the liver and can be used by the heart and skeletal muscles, kidney cortex, and in most species, the central nervous system to partially replace glucose as an energy carrier.However, in horses, the ability to form ketone bodies during energy deficiency is very limited. If horses enter a negative energy balance and cannot meet their energy needs either through more efficient fibre digestion or through proteolysis (the breakdown of protein for energy), this is highly problematic for their metabolism.
In horses, the fatty acids released from adipose tissue during lipolysis are re-esterified into triacylglycerols in the liver, which increases the risk of hyperlipidaemia. This can lead to the death of the animal, especially in cases of extreme starvation diets, such as those still commonly practised for obese laminitic horses. This kind of hyperlipidaemia does not occur in other species because they can efficiently use the broken-down fatty acids for gluconeogenesis. Therefore, oils should not be included in the energy balance for horses. They serve as building blocks for processes like tissue regeneration, the synthesis of various hormones, or even organ protection, but not as energy carriers!
Claim: “500g of oil per day is perfectly fine”
This value is based on theoretical calculations as follows: Hay typically contains 1-2.5% crude fat in our regions. A healthy horse, when fed ad libitum (free access to food until satisfied), consumes an average of 2-3% of its body weight in hay, which equates to 2-3 kg per 100 kg of body weight. For a horse weighing 600 kg, which is the standard reference used in feeding calculations, this amounts to about 12-18 kg of hay per day. Studies show that horses can eat up to 25 kg of hay per day as the absolute maximum.
Nutritional values are always expressed on a dry matter basis, so the amount of hay consumed must first be converted into dry matter before the crude fat content can be calculated. Hay typically contains around 10% moisture content. If we convert 25 kg of hay, this results in 22.5 kg of dry matter. With a maximum crude fat content of 2.5%, it can be assumed that a horse consuming 25 kg of hay per day would ingest approximately 562.5 g of crude fat. It’s important to note: we are talking about fats that are bound within the hay, gradually entering the digestive system over a 24-hour period, not as “oil slicks” in the concentrate feed. These fats, when added to concentrate or other carriers, must first be broken down by peristalsis and then emulsified, whereas in plants (such as hay and grass), fats are already contained in tiny fat micelles in seeds or as individual fat molecules within plant cells.
The efficiency of fat digestion is linearly dependent on the size of fat droplets. The smaller the fat droplets, the larger the surface area in relation to their volume, making the digestion process more efficient. This is because the lipases in a lipase-colipase complex can position themselves between the emulsifier molecules on the surface of the fat droplet. The optimal diameter for this process is between 100-1,000 nm (nanometres).
These fat droplets are formed from added dietary fats as they are broken down by peristalsis. The process is more efficient the longer it is carried out. You can easily try this yourself by adding water and oil to a glass and then adding a bit of dishwashing liquid. If you shake the glass briefly, you will notice that, once you set it down, relatively large fat droplets rise back to the top.
The longer you shake the glass, the smaller the fat droplets become, until they are so tiny that their specific gravity is no longer enough to make them rise, resulting in a homogeneous, milky liquid. The human small intestine has about 8 hours to peristaltically process dietary fat, breaking it down into well-digestible small fat droplets. In contrast, the horse’s small intestine only has 45-90 minutes for this process. The more fat that is added to the diet, the greater the risk that these fats will not be sufficiently emulsified, especially since the fats already present in the food must also be digested.
Moreover, the monoacylglycerides and fatty acids resulting from fat digestion must be transported conjugated with bile detergents in the form of mixed micelles. These micelles are star-shaped structures made up of hydrophobic and hydrophilic molecules, with a diameter of 3-5 nm. In this way, the emulsifiers are not fully available to make the fat droplets digestible, as they are also needed to transport the digested fats to the intestinal wall, where they are absorbed. This occurs in an organism like the horse, whose bile system is not designed to handle large amounts of fat and therefore has naturally limited emulsification capacity.
The ability to digest fat depends on several factors: how much fat in total enters the small intestine per unit of time and in what form these fats are already dissolved.
From a digestive physiology standpoint, it makes a huge difference whether 500g of fat is distributed across one or two meals as pure oil into the small intestine, or whether it is gradually released over a 24-hour period in pre-processed form as fats from roughage. This is comparable to whether I, as a human, consume my fats either dissolved in my food daily, as we all do—for example, butter on bread, cheese, sausages, meat, fats in fried potatoes, cream in pasta sauces, etc.—or whether I consume the same amount of fat (on average, a human consumes 80-90g of fat per day) in one portion, such as a cup of olive oil or a third of a block of butter. The often quoted 500g is, therefore, a theoretical value, originally not meant to indicate the amount that should be added to the feed bowl, but to show what a horse can maximally compensate in its normal diet. Since, even under experimental conditions, it is impossible to create a fatty acid deficiency in horses (because the hindgut microbiome is a net producer of essential fatty acids), such recommendations for supplementation should be critically questioned.
Regarding oil feeding, we also have a video on our website where we explain everything in detail. Feel free to check it out: https://sanoanimal.com/2020/12/04/video-olfutterung-sinnvoll-oder-nicht/
Oil Feeding for Horses: What Does Current Literature Say?
There isn’t just one study, but several dozen that have examined various aspects of oil feeding. Oil feeding in horses only began about 20 years ago, driven primarily by the feed industry, which was looking for alternatives to (flaked) grains, as an increasing number of horses were reacting to grain feeding with conditions like EMS, insulin resistance, and laminitis.
Horses are not equipped to handle such large amounts of sugar—especially when combined with the limited exercise they typically get. In order for owners to still have something (sugar-free) to feed, the feed industry has shifted towards offering structural roughage feeds as well as fats and proteins as substitutes.
Instead of addressing the underlying problem—that most horses in our current management conditions suffer from overfeeding combined with underwork. Whether it’s PSSM or insulin resistance – feeding fat- and protein-rich meals to such horses primarily satisfies the owner’s need to put something in the feed bucket. However, this has little to do with equine-appropriate nutrition.
From these fat-protein mueslis, the addition of oil to the daily feed ration developed, with the message from feed manufacturers that horses urgently need the energy from the feed, as they supposedly cannot get enough energy from hay alone. At the same time, however, these same horse owners are afraid of feeding their horses more hay, fearing that the horse might become overweight as a result.
Fats and proteins are the perfect food for carnivores. They primarily consume proteins and fats from their diet, which is what their metabolism is evolutionarily adapted to. In contrast, the horse, as an herbivore, consumes oils and fats in small amounts, as it is a roughage feeder, meaning it digests structural carbohydrates. Oils and fats are primarily found in seeds, which are consumed in small quantities as part of a species-appropriate diet.
Assuming that average hay contains around 2.5% crude oils and fats, a normal hay intake for a warmblood horse (ad libitum) of 15-20 kg would result in the intake of 375g to 500g of crude oils/fats over a 24-hour period. 500g of crude oils/fats per day is already the maximum recommended by the GfE (Society for Nutrition of Horses) for horse feed (this recommendation includes roughage as well! Unfortunately, many so-called feed “experts” tend to forget this…). This amount is very low when calculated relative to the horse’s weight, and that’s because horses are evolutionarily not adapted to digesting larger amounts of oils/fats.
Horses may tolerate oils to a certain extent – just as they tolerate starch, which they don’t actually need as an energy source. However, this does not justify the rationale behind oil feeding, as virtually all horses in our management conditions already have a significant energy surplus from being fed hay ad libitum (to satiety), which is not adequately burned off through work.
It can be assumed that hay contains an average of 8 MJ of energy per kg (with fluctuations between 7 and 9 MJ per kg). A 600 kg warmblood has a maintenance requirement of about 80 MJ, and if it is worked normally each day, an additional 8-12 MJ is required. If this horse is fed an adequate amount of hay (12-15 kg), it would receive 96-120 MJ of energy, while its requirement is only about 80-92 MJ. This means that the horse already has an energy surplus just from hay feeding alone.
Increasing the energy in the feed with oils is rather counterproductive in this context, especially since the energy extraction from oil is not a priority in the horse’s metabolism. Studies on performance improvement through oil feeding are particularly contradictory, with many actually showing a decline in performance from oil supplementation, rather than any improvement.
Prof. Coenen (author of the famous Meyer & Coenen “Horse Feeding”) himself is against supplementing oils in the feed ration, as he believes that:
1. All oils are imbalanced and do not match the optimal fatty acid profile of roughage
2. About 15% of the concentrate feed ration with added oil ends up in the large intestine, where it becomes toxic to the gut flora
3. Oils are not needed by horses, as they are sufficiently provided for by their roughage
4. No study has been able to demonstrate a deficiency of oils in horses.
(This was presented in a lecture at the Horse Feeding Seminar 2015 in Leipzig, as well as in personal communication at the same seminar.)
For more information on the scientific understanding of oils/fats in feeding, refer to R.J. Geor, P.A. Harris, M. Coenen’s “Equine Applied and Clinical Nutrition” (Saunders/Elsevier 2013) and D. Frape’s “Equine Nutrition and Feeding” (Wiley-Blackwell, 2010 or 2013), including the referenced original studies, as well as scientific databases such as PubMed: https://pubmed.ncbi.nlm.nih.gov/.
It is interesting, however, that one constantly has to provide scientific justification for why something should not be fed, while the claim that something is good as feed for horses is immediately accepted without question.
🇬🇧 We are an experienced team of therapists specializing in feed consultation and integrated therapies for horses. With extensive experience in treating metabolic issues, we focus on natural, species-appropriate feeding and proven naturopathic remedies to enhance your horse's health. Benefit from our expertise to ensure the well-being of your horse.
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