Adding fats and oils to your horse’s diet is a great way to increase calorie supply without relying on grains and high-NSC feeds.
Fats can be added to the diets of underweight horses as weight gain supplements. Oils also provide cool energy to support exercise performance, weight maintenance and gut health.
High-fat feeds are typically made with rice bran, ground flax, or vegetable fat. Oils such as canola, soybean, flax, or camelina oil are also popular options for horses.
All oils and pure fats provide the same amount of caloric energy per gram. However, not all oils are equal in terms of how they influence processes in the body.
When deciding which fat supplement to add to your horse’s diet, equine nutritionists also care about the fatty acid composition of the feed. As this article will discuss, certain fat sources have advantages over others.
Fat in the Equine Diet
Wild horses evolved to graze for up to 16 hours per day on fibrous forages on vast low-quality grasslands.
On a forage-only diet, most of the horse’s energy needs are met through hindgut fermentation of fibre, with only small amounts of fat present in the diet.
Hay and pasture typically contain 1 – 3% fat on a dry matter basis. This means that an average-sized horse consuming 10 kg (22 lb) of forage only gets 100 – 300 grams of fat in their diet per day, and not all of this will be absorbed in the gut.
Although not a significant part of the horse’s natural diet, there are some considerable advantages to supplementing your horse with fats in modern management settings.
Grains vs. Fats
Commercial feeds for horses with high calorie needs often contain grains or grain by-products as energy sources. Examples of grain-based ingredients include:
- Ground corn
- Ground wheat
- Corn germ meal
Although grains add calories to the diet, they are also high in non-structural carbohydrates (NSC) such as starch and sugar.
Horses have small stomachs and are not well-suited to digest large grain-based meals. Undigested starches and sugars spill over into the hindgut and cause gut health issues, such as dysbiosis, gastric ulcers and hindgut acidosis.
Oils and fats are often a better option than grain-based feeds to increase the calorie density of the diet because they provide calories without starch and sugar.
Replacing grain with fat sources can help to protect the health of your horse, reducing the risk of digestive issues or metabolic dysfunction.
Benefits of Feeding Fat
There are several additional benefits to meeting the energy requirements of performance horses from fat instead of carbohydrates. These benefits include:
- Reduce heat from digestion: The digestion of fats generates less heat than protein and carbohydrate digestion. Fats are a great option to provide cool calories for performance horses or for horses in hot climates.
- Spare glycogen: Glycogen is the preferred energy source for contracting muscle. However, horses performing low-intensity exercise can adapt to using more fat for energy. Horses in high-intensity exercise may still require some starch and sugars in the diet to restore glycogen stores post-exercise. 
- Improve performance: In some studies, horses adapted to added fat had faster race times, delayed fatigue, and reduced heart and respiratory rate. This could be due to reduced dry matter intake and therefore less gut fill, glycogen-sparing and reduced metabolic heat. 
- Decrease tying-up: High-starch diets contribute to high frequency of tying-up episodes. Decreasing starch and using fat for energy can help reduce the risk of recurrent exertional rhabdomyolysis. 
- Promote calm demeanour: Research suggests that replacing starch with fat reduces startled behaviour and reactivity to new noises and visual stimuli in horses. 
The fats found in your horse’s forage, feed and supplements are mostly in the form of triglycerides. These are molecules that consist of three chains of fatty acids bound to a glycerol backbone.
Fat digestion occurs in the small intestine, which is the area of the horse’s gastrointestinal tract that food passes through after it leaves the stomach.
In the small intestine, digestive enzymes and bile acids break down triglycerides into fatty acids and glycerol. These components are absorbed by the intestine and re-made into triglycerides to be delivered throughout the body.
Most mammals have a gall bladder which stores bile acids and releases them into the small intestine when a meal is consumed.
Horses do not have a gall bladder. Instead, they synthesize bile acids in the liver and continuously secrete bile into the small intestine.
Fat is a particularly efficient energy source for the horse because its digestion does not produce a lot of heat.  In contrast, fibre fermentation in the hindgut results in significant energy loss to heat production.
Research shows that horses on a high-fat diet had a 14% decrease in body heat production, meaning that more energy was available to fuel athletic performance and other metabolic functions. 
Although horses have evolved to subsist on low-fat, high-fibre diets, they can adapt to a high fat intake.
Studies show a linear relationship between the amount of fat in the diet and the amount absorbed. This means that as the fat content of the diet increases, the horse is able to absorb more fat up to intakes of over 2 kg per day. 
Fats in forages and grains are less digestible than those in added fats, such as oils or high-fat concentrates. This means that more of the fat is available for the horse to use as energy when provided in the form of fat supplements.
Fat molecules in forages and grains are found within the plant cell, which is surrounded by a rigid cell wall. This structure makes it harder for enzymes to access the fat and break it down into fatty acids for absorption.
How to Feed Fat to Horses
Because fats do not make up a significant portion of the horse’s natural diet, horses need to be slowly adapted to added fat sources.
Oils can be top-dressed on the current diet, mixed in with forage cubes/pellets or even poured directly onto your horse’s hay. Start with feeding 30 ml (1 oz) of oil daily and increase as needed by half an ounce (15 mL) every three to four days.
If you are starting your horse on a high-fat feed, introduce 25% of the desired feeding rate for three to four days and increase gradually after that.
This gives your horse’s body time to increase the production of digestive enzymes and adjust to handle higher fat intake.
Note that if you are giving your horse a high-fat commercial feed below its recommended feeding rate, you will be undersupplying vitamins and minerals. Horses on a high-fat diet should have adequate levels of all micronutrients, but especially antioxidant vitamins and minerals.
Monitoring for Changes
Introducing fat supplements to your horse’s diet too quickly can cause gut problems. If fats are not fully absorbed in the small intestine, they can reach the hindgut, altering gut microbial populations and impacting fibre digestion.
Monitor your horse’s feces for changes that suggest the fat was introduced too quickly: 
- Sheen on well-formed fecal balls
- Greasy or grayish appearance
- More abundant feces
- Loose feces
If you notice these signs, reduce the amount of oil or fat temporarily until your horse’s bowel movements return to normal. The fat sources can then be re-introduced at a slower rate.
How Much Fat to Feed your Horse
The amount of fat to add to your horse’s diet depends on their calorie needs and the digestible energy content of the rest of their diet.
All oils provide roughly 9 kilocalories (kcal) of energy per gram (mL) of oil. In comparison, carbohydrates and protein provide 4 kcal per gram, making fats a much more concentrated energy source.
If you were to feed your horse 30 mL (1 oz) of oil, this would provide 270 kcal of energy.
Most horses should be fed a diet with less than 8% total fat content. However, heavily-exercised horses can be fed up to 20% or more of their digestible energy requirement from fat.