By Lydia Gray, DVM, MA, Staff Veterinarian/Medical Director, SmartPak
Nutrition is the foundation of a healthy, happy horse that performs to the best of its ability. This article outlines ten “keys” to help you better understand nutrition and put its power to use. From basic information like the importance of forage to more advanced topics like the difference between “digestibility” and “bioavailability,” there is something for everyone who makes decisions about the diet of a horse. The article is made up of ten different sections, or keys, each covering a separate but related topic within equine nutrition:
KEY #1—Forage is the basis of a horse’s diet
KEY #2—But . . . forage is incomplete nutrition
KEY #3—Over- and Under-Supplementation
KEY #4—Nutrient Requirements of Horses
KEY #5—The Digestive Tract: Parts & Purposes
KEY #6—The Six Classes of Nutrients
KEY #7—Factors Affecting Nutrient Requirements
KEY #8—Body Condition Scoring (and other measurements)
KEY #9—Some Tricky Definitions
KEY #1—Forage is the basis of a horse’s diet
The most basic requirement in a horse’s diet is long-stem forage. Ideally, this comes in the form of fresh grass. If grass is not available, free-choice grass hay is the next best choice. Keeping hay in front of horses at all times allows them to most closely mimic their natural grazing behavior. When this feeding arrangement is not practical, horses should receive at least 1% of their body weight each day in forage, divided into as many meals as possible. For a 1000-pound horse, this is about 10 pounds of hay per day by weight, not by volume (flakes).
Horses with Equine Metabolic Syndrome (EMS), Cushing’s Disease and certain other medical conditions may have restrictions on their grazing time to control the amount of sugar and starches in their diet. Pasture may also not be appropriate for horses prone to laminitis (founder), especially when the specific sugar fructan is high such as in the spring and fall.
KEY #2—But . . . forage is incomplete nutrition
Because grass is deficient in certain minerals and hay is deficient in certain vitamins and minerals, horses need more than just forage as their diet. However, when fortified grain is added to try and meet vitamin and mineral requirements, calories are also added, which some horses don’t need. In addition, these extra calories are usually from sugars and starches, which can be a problem for horses with health problems such as Equine Metabolic Syndrome (EMS) and Polysaccharide Storage Myopathy (PSSM).
Fortunately, there are a variety of ways to meet a horse’s nutrient requirements that don’t tie needed vitamins and minerals with calories. The simplest option is to provide your pasture horse with minerals or your horse on hay with a multi-vitamin/mineral supplement. If you provide vitamins and minerals with a protein source (amino acids), then you are feeding what’s known as a ration balancer. Fortified grain is the next link in the chain, because it provides vitamins, minerals, protein and energy. Finally, some senior horses and those with certain medical problems (such as airway disease or dental issues) thrive on what is called a complete feed, which is like having hay and grain in the same bag. That is, these products provide all the necessary vitamins, minerals, protein and energy together with a source of fiber, so additional pasture or hay is not absolutely required. It is important to read the label on each of these products and feed the correct amount:
- Mineral or Multi-vitamin/mineral supplement: 1 - 4 ounces
- Ration balancer: 1 - 2 pounds
- Fortified grain: 5 - 7 pounds
- Complete feed: 12 - 14 pounds
KEY #3—Over- and Under-supplementation
An active pleasure horse at his ideal weight receiving recommended amounts of forage and fortified grain may have all his nutrient requirements met. However, this situation is often the exception rather than the rule.
Scenario 1: A 15-year-old “Easy Keeper” on grass hay only
In this scenario, the owner has greatly restricted her overweight horse’s diet--including removing all fortified grain--to cause him to lose weight. Depending on the type, cutting and quality of the hay, it may be supplying this horse’s protein needs, but it is probably deficient and unbalanced for many vitamins and minerals. This is a case of “under-supplementation” in which a horse is not receiving the correct amount and ratios of the nutrients his body needs.
Scenario 2: A 5-year-old racehorse on 50/50 forage and grain diet
Now consider a young adult racehorse that undergoes intense exercise on a regular basis. In order to meet his extremely high energy demands, he is fed equal amounts of a grass/alfalfa hay and fortified grain, most likely a sweet feed. Not only is this horse “over-supplemented” with vitamins and minerals because they are included in his energy source (the sweet feed), but feeding such high amounts of grain may make him prone to ulcers, colic and even tying up.
Keys #2 and #3 refer to the nutrient requirements of horses. The minimum recommended daily amounts of protein, carbohydrate, fat, vitamins and minerals that horses are listed in a publication, described in the next key.
KEY #4—Nutrient Requirements of Horses
For many years, veterinarians, nutritionists, feed manufacturers, teachers, students and horse owners relied on the fifth edition of the National Research Council (NRC) Nutrient Requirements of Horses, published in 1989. Fortunately, an updated version was published in 2007, making this valuable reference for the horse industry much more current. (Available for purchase at https://www.nap.edu/catalog/11653/nutrient-requirements-of-horses-sixth-revised-edition)
The purpose of this NRC publication is to review and summarize the existing scientific literature regarding the nutrition and feeding of horses as it relates to nutrient requirements of the different physiological classes: foals, weanlings and yearlings; adult horses in various levels of work; and breeding animals. The committee that prepared the publication makes clear that their suggested values may not meet the needs of all horses in all situations and that adjustments may be needed for individual horses or to meet specific goals.
Contents of the NRC Nutrient Requirements of Horses:
- Fats and Fatty Acids
- Proteins and Amino Acids
- Water and Water Quality
- Feeds and Feed Processing
- Feed Additives
- Feed Analysis
- Feeding Behavior and General Considerations for Feeding management
- Unique Aspects of Equine Nutrition
- Donkeys and Other Equids
- Ration Formulation and Evaluation
- Computer Model to Estimate Requirements
Chapter 12 covers the role of nutrition in some common equine medical problems, including nursing and orphan foals, “old age,” feeding management of horses in cold or hot weather, and nutritional management of specific disease conditions. Covered in this last section are hyperkalemic periodic paralysis (HYPP), tying up, PSSM, developmental orthopedic disease (DOD), laminitis, nutritional secondary hyperparathyroidism, ulcers, colic and recurrent airway obstruction.
Because reviewing and summarizing information about the digestive physiology of the horse was outside the charge of the Council, the next “key” will provide a brief description of the parts and purposes of the digestive tract so the information in the Nutrient Requirements of Horses is more helpful.
KEY #5—The Digestive Tract: Parts & purposes
The horse can be classified as a non-ruminant herbivore and a hindgut fermentor. Still, the digestive tract of the horse is similar to other species and is made up of the following parts:
- Mouth—food entrance, mechanical breakdown
- Salivary glands—food moistening, some carbohydrate digestion
- Esophagus—carries food to the stomach
- Stomach—protein digestion
- Small intestine (duodenum, jejunum, ileum)—further carbohydrate, protein and fat digestion; absorption of nutrients
- Liver and pancreas—aid in carbohydrate, protein and fat digestion and absorption
- Large intestine (cecum, colon, rectum)—water and electrolyte absorption
- Anus—waste exit
Relative to a horse’s overall size, its stomach is very small, making up less than 10% of its entire digestive tract and holding only about two gallons. On the other hand, the colon makes up almost half of the horse’s digestive tract and can hold about 15 gallons. These two differences are important and should affect the way a horse is fed. The horse’s small stomach means it was designed to eat continuously or at least small meals frequently. The large colon is actually a fermentation “vat” staffed by bacteria that ferment fiber the horse is unable to digest itself, manufacturing nutrients like energy and B-vitamins. Putting additional hay into this fermentation “vat” during especially cold winter days is an excellent way to heat your horse from the inside out.
Now that we know how the different parts of the horse’s digestive tract function to digest and absorb nutrients, it’s time to explore these nutrients themselves. Key #6 defines each nutrient, explains its roles in the body, and provides some recommended amounts.
KEY #6--The Six Classes of Nutrients
Nutrients are divided into six categories: water, protein, carbohydrate, fat, vitamins and minerals. All are essential to life, but water is usually listed first because animals can survive without the other nutrients longer than they can survive without water, as 70-75% of the body consists of water. The two main functions of water are as a component of metabolism and a factor in body temperature control. Generally, horses drink between 5 – 15 gallons per day, depending on their size, the environment, their workload and other factors. The best advice is to always have clean, drinkable water available at all times.
Although protein is listed here as a nutrient, horses actually have a requirement for amino acids, the building blocks of protein, rather than for protein itself. According to the NRC’s Nutrient Requirements of Horses, the adult horse in minimum work that weighs just over 1000 pounds needs 540 total grams of protein in its diet per day. Another way of saying this is that the diet should contain 8% protein. Next to water, protein is the most abundant substance in the body, making up not just muscle but also connective tissue such as skin, hooves and hair, as well as enzymes, hormones and other substances. Proteins are commonly made of 20 amino acids but only 10 of these are considered essential, meaning they must be supplied in the diet because the animal cannot make them itself. Lysine is considered the first “limiting” amino acid because if it is not present in adequate amounts, the body’s ability to manufacture proteins is limited.
Carbohydrates mainly serve as an energy source for horses, but their fiber component is also necessary to keep the large intestine moving and functioning properly. There are many ways to categorize carbohydrates and all of them are somewhat confusing. One method is to divide them between structural and non-structural carbohydrates (NSC). Structural carbohydrates include the completely indigestible lignin--which passes through the horse’s digestive tract unchanged-- as well as cellulose and hemi-cellulose, insoluble fibers that can only be digested by bacteria that live in the colon. Non-structural carbohydrates include single sugars like glucose (monosaccharides), double sugars like lactose (disaccharides), medium-length sugars like fructoligosaccharide (also known as FOS), and longer sugars like starch (polysaccharides).
Interestingly, a requirement for fatty acids in the horse has not been established, although nutritionists suggest the diet contain at least 0.5% linoleic acid, an omega 6 fatty acid. Recent studies have shown that supplementing with omega 3 fatty acids may down-regulate inflammation in the body, especially in the skin and respiratory system. Fat is necessary for cell membrane health, as a carrier for fat-soluble vitamins out of the GI tract, and as a precursor to prostaglandins. However, research has shown other benefits to adding fat to the diet, up to 20% on a dry matter basis. Because it is more energy-dense than carbohydrates, fat can provide additional calories to the hard-working horse or the thin horse that needs to gain weight. Fat as an alternative source of energy to the simple carbohydrates found in grain leads to less excitability and improved efficiency.
Vitamins are organic elements required in small amounts in the diet by the body for essential metabolic functions to prevent overt signs of disease. They are divided into two kinds: the fat soluble A, D, E and K and the water soluble B-vitamins and Vitamin C. Because the horse can make some of the vitamins it needs (some B-vitamins, C, D and K), not all are required in the diet under normal circumstances. However, senior horses with less efficient systems or any horses undergoing stress from injury, illness, transport, or GI conditions that may interfere with normal gut flora may require supplementation.
Minerals are inorganic elements recognized to perform essential functions in the body and must be present in the diet. They are also divided into two kinds: the macrominerals which are required in larger amounts than the micro or trace minerals. Macrominerals include Sodium (Na), Chloride (Cl), Calcium (Ca), Phosphorus (P), Magnesium (Mg), Potassium (K) and Sulfur (S). Microminerals include Cobalt (Co), Copper (Cu), Iodine (I), Iron (Fe), Manganese (Mn), Selenium (Se) and Zinc (Zn). Other minerals of interest include Chromium (Cr), Fluorine (F) and Silicon (Si).
KEY #7--Factors Affecting Nutrient Requirements
The NRC recognizes the following physiologic classes of horses: growing animals, lactating mares, pregnant mares, stallions, working horses, and adult horses in no work. Within each class are several subclasses. For example, working horses are divided into light, moderate, heavy and very heavy exercise. These physiologic classes are determined by age and workload, with workload being either exercise or reproduction. However, there are many other factors affecting the type and amount of nutrients required by an individual horse, such as:
- Stress from training, competing and shipping
- Disease or injury
- Becoming a “senior” horse
- “Easy keeper” vs. “Hard keeper”
- Weather or environment
- Management or housing
- Quality of feedstuffs
Horses undergoing stress may need more B-vitamins while those recovering from disease or injury may require antioxidants like Vitamin E. Senior horses have been shown to benefit from added Vitamin C. Easy keepers may do best on hay that has either been analyzed for low NSC or soaked to remove excess sugar. Hard keepers may thrive on a fat-supplemented diet. When the weather turns cold, additional hay, not grain, will help horses stay warm and maintain weight. Some horses prefer their stall and individual turnout while others keep their ideal condition in group turnout on pasture. Finally, the quality of forage especially but also grain or supplements has a large impact on a horse’s weight, energy level and overall health.
Determining how much a horse weighs isn’t easy, but the next key provides some tools for estimating weight as well as body condition, a useful measurement to track.
KEY #8-- Body Condition Scoring (and other measurements)
In addition to knowing your horse’s normal vital signs (temperature, pulse, respiration and others), it’s a good idea to have a system in place that also allows you to monitor changes in his weight and body condition. Since it is fairly inconvenient to regularly weigh horses on a scale, there are several methods of estimating the weight of horses. The simplest is to use a commercial weight tape. Depending on the manufacturer and how close your horse’s size and shape is to the average horse, these tapes can be very accurate or they can be off by 100 pounds or more. Sometimes the best use of weight tapes is as a tool to track changes. That is, if your horse tapes 1000 pounds on November 1, then 975 pounds on December 1, then 950 pounds on January 1, then you know he is losing weight. A more accurate method of estimating weight is taking two measurements of your horse and plugging them into this weight formula:
Heart girth(in) X heart girth(in) X Length (in) = weight in pounds
The heart girth is the circumference of your horse’s barrel taken at the highest point of the withers and the length is the point of the shoulder straight back to the point of the buttock, half the distance from the corner to the tail.
In addition to estimating your horse’s weight and monitor changes up or down, your horse’s condition, or amount of fat cover, should also be estimated regularly. An excellent tool for this measurement is the Henneke Body Condition Scoring Chart, because it provides a standard scoring system for you, your veterinarian, your nutritionist and other health care professionals. The scale ranges from a “1” which is the thinnest to a “9” which is the fattest—a score of “5” is ideal for most breeds and disciplines:
8=very fleshy (fat)
9=very fat (obese)
Body condition score, nutrient requirements, over and under-supplementation . . . This article has thrown some pretty hefty words around. Key #9 provides definitions of some words frequently used in any nutrition discussion.
KEY #9—Some Tricky Definitions
Some words in nutrition are used interchangeably but they may not mean exactly the same thing. This section includes some commonly used and misused words and their definitions.
Nutrient—Any food component that is necessary for the support of life. Also, a chemical substance that nourishes, such as protein, carbohydrate, mineral or vitamin.
Ingredient—Edible material that may provide nutrients as part of food.
Food-Any material, usually of animal or plant origin, containing essential nutrients.
Feed-Food for animals.
Feedstuff-Any substance suitable for food, several foodstuffs are normally combined to provide a balanced diet. Any component of a diet that serves some useful function.
Diet-A regulated selection or mixture of feedstuffs provided on a continuous or prescribed schedule. A balanced diet supplies all nutrients needed for normal health and productive functions.
Ration-A fixed portion of feed, usually expressed as the amount of a diet allowed daily.
Digestibility—The percentage or proportion of nutrients in food available for absorption from the GI tract.
Bioavailability—The amount of a nutrient absorbed from the GI tract in a form the body can use.
Roughage—The primary foods for all herbivores existing under natural conditions, such food provides the major portion of their diet for most if not all of the year. Includes pasture, green chop, hay, chaff, silage and other forms. A bulky feed that has a low weight/unit of volume, high crude fiber content and low digestibility of nutrients.
Digestion-The preparation of food for absorption which may include mechanical, enzymatic and chemical actions. Overall function is to reduce food to a molecular size or solubility that will allow for absorption.
Absorption-Includes various processes that allow small molecules to pass through the membranes of the GI tract into the blood or lymph systems.
Appetite-a desire for food or water. Generally, a long term phenomenon in contrast to short term satiety. Refers to internal factors that stimulate or inhibit hunger in the animal.
Palatability-The overall acceptance and relish with which an animal consumes any given feedstuff or diet. A summation of many different factors sensed by the animal in the process of locating and consuming food such as appearance, odor, taste, texture, temperature.
Taste-To distinguish flavors between or among feed or water components.
Satiety-the condition of being fully satisfied with food. The opposite of hunger.
Hunger-The desire for food. The opposite of satiety.
All definitions from Basic Animal Nutrition and Feeding, third edition, Church DC and Pond WG, 1988 John Wiley & Sons, Inc.
This article covered a lot of ground! First we covered the importance of forage yet explained that forage wasn’t a complete and balanced diet. Then we discussed how easily horses can be over- or under-supplemented based on the use of a one-size-fits-all approach to grain. Next we introduced the NRC Nutrient Requirements of Horses, reviewed the horse’s digestive tract, outlined the six class of nutrients, and explored what factors might alter the requirements for certain nutrients. Finally, no discussion of nutrition is complete without covering body condition scoring and methods of estimating weight in horses. The article wraps up with definitions of some common nutrition words and the all-important Resource Section. By no means is this section complete, they are just a few of my favorites that I use regularly.
Nutrient Requirements of Horses. Sixth revised edition. National Research Council of the National Academies Press, Washington, DC 2007.
Understanding Equine Nutrition: Your guide to horse health care and management. Briggs, K. The Blood-Horse, Inc. Lexington, KY. 2007.
https://www.facebook.com/groups/1246635855537015/ Safergrass Facebook page for facts on pasture, fructans and feeding the insulin resistant horse.
http://counties.agrilife.org/smith/files/2011/06/hrg012bodyweight_2.pdf for the horse weight prediction equation or https://www.smartpakequine.com/content/horse-weight-calculator for a calculator that does the math for you!
https://equi-analytical.com/ for hay analysis.
https://extension.umaine.edu/publications/wp-content/uploads/sites/52/2015/04/1010.pdf for pictures and instruction on body condition scoring.
Reviewed and updated by original author in 2016.