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Feeding fat (oil) to horses

Fats and oils are made up of triglycerides that each consist of a glycerol molecule to which three long chain fatty acids are attached. Fats and oils are the same compounds nutritionally; oil at room temperature will be a free-flowing liquid, whereas fat at room temperature will be solid. Fats/oils are digested in the small intestine of the horse and are a concentrated source of dietary energy, providing approximately 2.25 times more energy than an equal weight of digested carbohydrates.


Forages for Horses in Iowa

Forages as hay or pasture make up the significant share of the daily intake of a horse. Horses are natural grass eaters with front teeth suited for biting off the grass. The molars chew and grind bulky feed, such as hay and coarse grains. A mature horse that is not working hard will eat 1.5 to 2 pounds of air-dry feed per 100 pounds of body weight. That would be 15 to 20 pounds of hay daily for a 1,000-pound horse. In Iowa, horses will require about 2 tons of hay per head per year plus summer pasture. When meeting nutrient requirements, carefully consider forage quality and nutrient content.

Digestive Anatomy and Physiology of the Horse

Horses are non-ruminant herbivores, meaning they eat mainly plant material. The horse’s gastrointestinal tract consists of the mouth, esophagus, stomach, small intestine and the highly developed large intestine composed of the caecum, large colon, small colon and rectum (figure 1).

The Mouth

Anatomical features of the mouth include the teeth, tongue and salivary glands. Digestion of feeds begins when food enters the mouth. The horse chews reducing feed particle size and mixing it with saliva to begin the digestive process. Saliva acts as a lubricant to provide easier passage through the esophagus and buffers acid in the stomach. Once swallowed the bolus of feed moves from the esophagus to the stomach.

The Stomach

The stomach of the horse is the smallest unit of the digestive tract with a capacity of approximately 2-4 gallons, comprising around 10% of the total volume of the horse’s digestive tract. The horse has the smallest stomach in relation to body size of all domestic animals. Due to the small capacity, smaller, frequent meals are recommended. The stomach’s main functions include mixing, storage and controlled release of feed into the small intestine; and secretion of pepsin to begin protein digestion. Very little absorption of nutrients occurs in the stomach. Once feed is released from the stomach it enters the small intestine.

Small Intestine

The horse’s small intestine is approximately 70 feet long, comprising 30% of the total digestive system. The passage of feed through the horse’s small intestine is rapid, moving at approximately 1 foot/min and delivering the digesta to the cecum in as little as 45 minutes after a meal. volume of feed consumed and rate of passage affect digestion and absorption of nutrients – larger volume and increased rate of passage will decrease digestion and absorption

In the small intestine a majority of non-structural carbohydrate (starch), protein and fat is digested by enzymes and absorbed. Starch is digested by amylase enzymes, oil is digested by lipase enzymes and protein is digested by protease enzymes. These enzymes, which are produced either in the pancreas or the small intestine, reduce starch into glucose, fats (oil) into glycerol and fatty acids, and protein into amino acids.

The digestion of oils and protein is extensive in the small intestine. The digestion of starch can often be incomplete due to the starch present in cereal grains being protected by the grains seed coat. If starch is not digested in the small intestine it will be delivered to the hindgut where it will be rapidly fermented by bacteria, causing lactic acid production and accumulation, hindgut acidosis and diseases such as colic, metabolic acidosis and laminitis/founder.


The hindgut of the horse comprises the cecum, large colon, small colon and rectum. The cecum consists of 12-15% of tract capacity and the colon 40-50% of tract capacity. The major functions of the hindgut are the microbial digestion (fermentation) of dietary fiber (structural carbohydrates primarily from forages in the horse’s diet). Important end-products of the fermentation are volatile fatty acids (acetic, propionic and butyric) which can serve as an energy source for horses fed mostly forages such as pasture or hay. Fermentation also produces methane, carbon dioxide and water, as well as most of the B-vitamins and some amino acids. Another function of the hindgut is water reabsorption.

The diet composition affects the makeup of the microbial population. When starch is delivered to the hindgut the starch fermenters (amylolytic bacteria) begin to rapidly ferment the starch, producing large quantities of lactic acid and volatile fatty acids (VFA). Because of the acidic nature of these products of fermentation, the pH in the hindgut begins to fall. A low pH favors pathogenic bacterial which can then contribute to serious diseases such as, laminitis or founder, colic, endotoxemia and metabolic acidosis.

Equine Digestive System
Figure 1. Equine Digestive System

Management Suggestions

  • maintain regular feeding schedule
  • base a feeding program on high quality forage – only feed concentrate to meet nutrient requirements not met by the forage
  • feed small meals, especially concentrates, in small amounts (< 4-5 pounds of concentrate/meal)
  • minimize the NSC levels in the concentrate while assuring adequate supply of energy and other non-calorie nutrients
    • Utilize highly digestible fiber and fat for increased calorie needs in performance horses, lactating mares and growing horses wherever possible
  • make feed changes slowly
    • when changing to a richer forage such as pasture or legume hay allow 7 to 10 days for adjustment of microbes in hindgut
  • follow a regular schedule for dental care and deworm

Protein and the Horse’s Diet

Dietary protein supplying amino acids is a necessary component of the horse’s diet.  Proteins are required for a multitude of key bodily functions. These include major components of muscle, enzymes, hormonal roles, the immune system, and transport of nutrients across membranes and in blood. Protein assists in tissue repair and growth, hence the amount of protein required in growing, pregnant, lactating or heavy exercising horses is increased. Proteins are made from various combinations of amino acids, and consist of carbon, hydrogen, oxygen, nitrogen and some Sulphur atoms. In horse feeds, protein is expressed as crude protein (CP) because it is an indirect or ‘crude’ estimation of that feed’s protein content. Digestible protein (DP) would be more ideal to describe a particular feed, but there is insufficient research data for DP values for horses.

Energy, Feed Ingredients and the Horse

Energy is supplied to the horse via the diet but fundamentally energy is not a nutrient. Horses need energy to carry out their body’s essential daily functions, including the digestion and absorption of food, activity, growth and reproduction. The total energy contained in a feed is called gross energy (GE). The GE of a feedstuff is not a good indicator of the energy available to a horse from a feedstuff. Thus, with horses, we use digestible energy (DE) which is GE minus the energy contained in feces. Digestible energy is expressed in Mcal which is a multiple of the unit of energy calorie. Table 1 lists the DE of common feed ingredients in horses’ diets. In general, cereals provide more DE than hay. Vegetable oils contain more energy than cereals (~2.5 times as much DE as corn and three times as much as oats).

Energy is primarily derived from carbohydrates and fats/oils in the diet: Carbohydrates are commonly divided into non-structural carbohydrates (NSC) and structural carbohydrates. NSC’s consists of starch and simple sugars. Starch constitutes around 40% of the total weight of oats, 60% of the total weight of barley and 70% of the total weight of corn. The remainder of these grains are made up by structural carbohydrates, protein and some fat.

Starch is primarily digested in the small intestine of the horse by enzymes. The starch is broken up into single glucose units which are then absorbed by the horse and used as a source of energy. Any starch that is left undigested passes through the small intestine, rapidly fermented by bacteria in the hindgut and can potentially cause hindgut acidosis leading to digestive issues. Processing cereal grains involves changing the structure of the feed to enhance the efficiency with which starch is digested in the small intestine.

Processing includes cold processed (rolled, ground, cracked) or hot processed (crimped, pelleted, extruded, steam-flaking, and micronized). Rolling passes the grain between a closely fitted set of smooth rollers. Crimping uses rollers with corrugated surfaces. Steam-flaking subject’s grains to steam for various periods of time. Steam-flaked grains are usually less dusty than dry-rolled grains. Pelleting uses ingredients which are ground and then steam heated. Next the mash is pushed through a pellet die to obtain the desired size, cooled and dried to prevent mold growth. The ingredients for extruded feeds are grinded, mixed and the resulting mash is cooked using a combination of high temperature and pressure. Extrusion die nuggets or cubes are then cooled and bagged. Micronizing heats the grain using infrared heat until all moisture is vaporized. This ruptures the endosperm of the grain causing the starch to be reconfigured. Immediate flaking further reconfigures (gelatinizes) the starch which enhances the digestibility and nutritional value of the feed. The goal with all the types of processing is to increase the digestibility of the feed ingredient in the small intestine, thus lowering the amount of indigested starch entering the hindgut. Safety and energy value of the feed greatly increases with processing.

Horse grain
Figure 1. Processed pelleted horse feed

Structural carbohydrates are also referred to as fiber or roughage are found in feedstuffs such as hay, pasture, soybean hulls and sugar beet pulp. Components include cellulose, fructan, hemicellulose and pectin. Bacteria in the hindgut ferment structural carbohydrates producing volatile fatty acids which are absorbed from the hindgut of the horse and used as a source of energy, especially at rest. Crude fiber listed in Table 1 and is a measure of the quantity of cellulose, and other structural carbohydrates. All of the fibrous feedstuffs have higher crude fiber then the grains listed. Structural carbohydrates provide the bulk necessary for proper peristaltic action in the horse’s intestinal tract.

Feeding horses depends to a large extent on the relative proportions of the energy sources provided to the horse and the feedstuffs used. Energy required per day is influenced by body condition; physiological status (age, breed, production); duration and repetition of work; environment, metabolic efficiency and presence of any nutritionally related disease. 

Table 1. Nutrient Composition in Common Feed Ingredients Fed to Horses


DE, Mcal/lb

Starch, %

NSC, %

Fat, %

Crude Fiber, %



















Wheat Middlings    






Beet Pulp






Soybean hulls      






Grass Hay






Grass Pasture 






Legume Hay






Mostly Grass Hay






Mostly Legume Hay






Values from Equi-Analytical




Calcium and Phosphorus – Two Important Macro Minerals for the Horse

Calcium (Ca) and phosphorus (P) comprise around 70% of the mineral content of a horse’s body. The majority of Ca is found in teeth and bones. Calcium’s major role is to provide bone strength but it is also necessary for blood coagulation, temperature regulation, enzyme activity regulation, neuromuscular functions as well as energy generation. Phosphorus works with Ca to give strength to bone. It also has a role in energy metabolism, cell membranes, and buffering fluctuations in pH.

As show in Figure 1, Ca is typically 6 times higher than P in legume pasture or hay. Grass hay usually have more Ca than P, while cereal grains are inherently low in Ca and high in P. Calcium supplements include Ca carbonate (limestone) and dicalcium phosphate. Phosphorus is present in most hays and high in cereal grains (figure 1). Dicalcium phosphate is a common Ca and P supplement. Calcium and phosphorus must be provided in the horses’ diet in the correct quantities and ratio to one another. Requirements for Ca ranges from 0.3 to 0.8% and P ranges from 0.2% to 0.5% of the total diet. The upper safe levels are 2% of the diet for Ca, and 1% of the diet for P. It is important that the dietary Ca: P ratio be maintained within the range of 1:1 to 3:1 for growing horses and between 1:1 and 6:1 for mature horses.

When there is a dietary Ca or P deficiency, the skeleton may mobilize Ca and P to maintain plasma concentrations and non-skeletal functions. Either a deficiency or excess of Ca or P can result in skeletal diseases. Over-supplementation with Ca rich supplements is the most common cause of Ca excess. The ratio of Ca:P can be less than 1:1 when insufficient Ca is fed, when excess P is fed, or when more P than Ca is fed. If excess dietary P is fed, it may bind Ca and prevent its absorption that may result in a Ca deficiency. The following situations can result in a less than 1:1 ratio.

  1. If a diet containing a high proportion of unfortified cereal grains is fed, there may be more P than Ca. Commercial grain products always balances the Ca and P in the product.
  2. A diet consisting of a high percentage of wheat bran. Bran contains a high concentration of P and diets with a high percentage of bran commonly have a Ca: P ratio that is less than 1:1.
  3. A diet that contains an excess of a P containing mineral supplement.
  4. A diet based on a pasture which has a low Ca and normal to high P concentration.
  5. Horses grazing pastures in a region with P deficient soils or are fed hay from a region with P deficient soils can result in a dietary P deficiency.
Calcium and phosphorus in selected feedstuffs for horses

Figure 1. Calcium and Phosphorus (%) in Selected Feedstuffs

Calcium and P are two very important macro minerals required by horses. The ratio of Ca:P is as important as the total amount fed. Generally, if one feeds a commercial grain with either a grass or legume hay the Ca:P ratio should be met. The ratio is less than 1:1 with overfeeding Ca or P supplements, feeding excess bran or the soil has excess P resulting in forages with excess P.  Always look at the total ration when assessing if the nutrient requirements are being met for a horse.