Basic Concepts in Nutrition
The options and opinions on what to feed your horse are numerous. My goal in this article is to review the differences between the "simple-stomached" creatures (such as ourselves, dogs, cats, etc.) and the horse. In addition, we will take a look at how domestication has impacted the horse's digestive system and subsequently how that impacts their general health. We will also review the basic components of nutrition and the concept of balancing a ration based on an individual horse's requirements and the factors that can affect those requirements.
The horse is considered a "post-gastric fermenter," meaning that after the simple stomach and small intestine, a large portion of the horse's digestion comes from the fermentation of fiber within the later portions of the digestive system. Actually, about 60% of the energy a horse gets from his food can come from volatile fatty acids produced by the bacterial fermentation of fiber within the large intestine (this is how most digestion occurs in the cow); more on that later.
The nutritional Calorie (Cal with a capital C) is a designation of energy and is typically referred to as "kcal," which is the unit used in human food labeling. One Calorie or kcal is the amount of energy required to raise the temperature of 1 kilogram of water 1°C. The term Mcal is equivalent to 1,000 kcal.
The bulk of the average horse's feed ration is hay (as it should be). The nutritional quality of hay can vary tremendously between a pure grass hay such as timothy, which has a digestible energy content of about 1.8 Mcal/kg and a crude protein content of about 7.5%, and a high-quality legume hay such as alfalfa, which has a digestible energy content of about 2.2 Mcal/ kg and a crude protein content over 15%.
The nutritional content of hay can also be affected greatly by weather conditions at the time of harvest and other processing factors. The best method for determining the nutritional content of a particular load of hay is to have a representative sample analyzed.
The nutritional requirements of an individual horse vary with stage of growth, sex, pregnancy and lactation, and other outside factors such as healing and recovery from illness, climate, and exercise. For example, the average 1,100-pound (500-kg) mature gelding requires 16,400 kcal (Calories or 16.4 Mcal) with 660 grams of crude protein for daily maintenance. This horse would require 20 pounds (9.1 kg) of our average timothy hay or 16.3 pounds (7.4 kg) of the average alfalfa hay for daily maintenance.
A mare of the same weight that just foaled and is lactating requires 28,000 kcal (28.0 Mcal) for daily maintenance, and a performance horse of the same weight working intensely (a high-end three-day event horse in training) would require 32,000 kcal (32 Mcal) for daily maintenance. This mare would need 34.2 pounds (15.6 kg) of timothy or 28 pounds (12.7 kg) of alfalfa hay; the performance horse would need 39 pounds (17.8 kg) of timothy or 32 pounds (14.5 kg) of alfalfa hay to meet his energy needs. Note that these values are approximate. Unfortunately, a horse can only eat about 3% of its body weight in dry matter per day (about 30 pounds for a 1,000-pound horse), so the performance horse and possibly the mare would need a higher energy density feed (such as grain) to meet the energy needs.
A general rule of thumb for daily maintenance is to feed 2% of the horse's body weight in pounds of average quality mixed grass hay, so it follows that the average 1,000-pound horse would require approximately 20 pounds of mixed grass hay a day to maintain weight. By now it's clear that doing any of this with remote accuracy requires weighing both the horse and the feed. In place of a full-size horse scale, the weight tape is fairly accurate, but electronic scales suitable for use on large animals are available and are becoming more commonplace in larger stables. Some type of scale should be acquired for the feedstuff--the weight of a flake of hay can vary tremendously, and it is very hard to guess (the same is true for many grain products). You really should feed grain rations based on weight, as well.
Protein is composed of molecules called amino acids. There are 22 amino acids used in the construction of proteins. From a nutritional standpoint, the proteins within a foodstuff are broken down by the digestive system into individual amino acids and are absorbed. From there the amino acids are used as needed to construct new proteins, such as muscle tissue. In extremely nutritionally deficient states, the proteins can be used to produce energy, but at the expense of normal protein tissues (e.g., heart and other muscles).
A note on protein--it is not a certain percentage of protein, but an actual gram amount, that should be used to meet requirements. It should also be noted that a certain quality of protein is necessary in order for it to be utilized. The amino acid lysine is the so-called rate-limiting amino acid for the horse, meaning there must be a certain minimal amount of lysine (23 grams per day for our 1,100-pound gelding) in order for the dietary protein to be utilized properly. Feeding a certain percentage of protein grain concentrate (12% or 16%) really does not say very much. You need to be aware of the total amount of protein you are feeding and understand the actual quantity (not percentage) contained in the daily hay and grain ration compared to that individual horse's daily requirement--for example, the 665 grams/day of our 1,100-pound adult gelding needs.
Carbohydrates can be simple (also called soluble carbohydrates or monosaccharides), such as glucose and various other sugars, or complex, such as starches, fiber, and the less soluble portions of grains. The grains have both soluble carbohydrates (the inner parts of the seed or kernel) and insoluble carbohydrates (the outer parts of the seed and remainder of the plant). The grain products are known as concentrates, and are composed of the relatively more soluble carbohydrates than roughages.
The digestibility of the grains is also affected by the processing. For example, crimped/rolled oats or cracked/ground corn have soluble carbohydrates that are more available to the digestive process than their "whole" counterparts. Most of the soluble carbohydrates are broken down and absorbed in the small intestinal portion of the horse's gastrointestinal system, with the insolubles passing on to the colon and cecum to be fermented by bacteria to produce volatile fatty acids. The glucose and volatile fatty acids are then used by the horse's biochemistry to produce energy or are stored in various tissues as fat reserves for later energy production. More on the fermentation process later.
Fats are also broken down and absorbed by the small intestine. From there the fats can either enter the biochemical energy production pathways or be stored for relatively long-term energy storage. Recent research has shown that the fats can make up to 10-20% of a horse's daily energy requirements and be well utilized. In fact, horses afflicted with muscle diseases (such as tying-up or glycogen storage disease) can benefit significantly from lowering the soluble carbohydrates (grain ration) in their diets and increasing the amount of energy that's provided by fat. Fats contain two times the energy production ability on a gram per gram basis compared to soluble carbohydrates or protein.
Vitamins are important to consider in the horse's ration, as many factors can decrease the vitamin content of horse feed. Some vitamins are sensitive to sunlight, heat, and oxidation (especially vitamins E and A). Hay stored for a year or longer, and hay that was rained on between cutting and baling, can have greatly decreased concentrations of these vitamins. In addition, the processing of many pelleted feed products involves heat and pressure, so it is important to know if the vitamins were supplemented, and if so how were they added. The most common vitamins added to horse feeds are A (important for reproduction), E (a natural preservative/antioxidant that helps ensure optimum function of the reproductive, muscular, circulatory, nervous, and immune systems), and H (a.k.a, biotin, which helps improve hoof and hair quality and is needed for the synthesis of fats, proteins, and glucose). Horses that don't have access to fresh grass during the winter months (or at all) have been shown to become significantly vitamin E deficient during the winter months.
The minerals are relatively stable (don't break down) in feedstuff processing, but the concentration of a particular mineral within a plant is proportional to concentrations within the soil. For example, it's well known that many areas of the United States produce selenium-deficient forage. It is important to know if you are in a geographic area that requires selenium supplementation (ask your local extension agent or have your soil tested). The most common minerals added to processed feeds are calcium, phosphorus, copper, zinc, and selenium.
Selenium deficiency can cause muscle disease in both adult horses and newborns. Check with your veterinarian or equine nutritionist regarding the supplementation of selenium, because the difference between the required amount and a toxic amount is very small. Be careful to avoid using three or four feed supplements that contain selenium; this can inadvertently cause toxicity. This holds true for all of the supplements--be aware of exactly what they contain and how much of it and how that stacks up against your horse's needs.
Water is the most important nutrient. The horse could live for weeks without ingesting any feed, but two to three days without water would cause severe illness or death. The average 1,000-pound horse drinks over 18 liters (4.5 gallons) of water per day. It goes without saying that a source of fresh, clean water should be available to the horse at all times. Intake will vary somewhat between individuals and will increase dramatically in hot weather, lactation, and/or if the horse is sweating a lot.
Electrolyte supplementation will increase water intake, but usually is not necessary unless the horse is sweating heavily. Sweat contains significant amounts of the major electrolytes: Sodium, chloride, potassium, and lower amounts of calcium and magnesium. These need to be replaced if the horse is sweating heavily.
Some horses will not drink water with electrolytes added to it, so I believe it is very important that if you are adding an electrolyte supplement to the water, that you also provide a bucket of water without the supplement. White salt (sodium chloride) should be available free choice at all times. The other electrolytes (potassium, magnesium, and calcium) are usually present in adequate amounts in the feeds to take care of normal sweat losses.
In climates where the winter is cold, you need to be careful to prevent the water source from freezing. It is quite likely that the slight rise in impaction colic cases we see in the winter months is associated with a decrease in normal water intake in some horses. It has been shown that offering room-temperature water when the ambient temperature is cold will significantly increase a horse's daily water intake.
The horse evolved as a grazing animal with a digestive system optimized to slowly ingest the daily ration of 100% grass over a 24-hour day. Two other factors I think are important are:
- Grazing is linked with almost continuous exercise; and
- The fact that grass is about 60% water.
Flash forward to the domesticated horse and we find many that stand in a stall over 20 hours a day and their grass (now processed hay) is only about 10% water. In addition, domestication puts many horses in a situation where they are eating only two meals a day. Many horses appear to tolerate the domesticated situation well, but I feel that some do not, and it is important to recognize this. I have cared for more than one "chronic" colic that responded well to splitting the feed ration into three to four feedings per day and making the effort to get them some time out on pasture.
There are other factors affecting digestion in the horse that are worth considering in this discussion. The fermentation that occurs in the hindgut of the horse is a very complex and somewhat delicate process that can be disrupted by many factors. Millions of bacteria in the gut have their own life cycles of digesting the complex carbohydrates of ingested hay, with the horse benefiting from the bacterial byproducts. If something occurs to "shock" or change the population of bacteria, the rate of fermentation changes along with the byproducts of the fermentation. This leads to the production of excessive amounts of gas.
In addition, disrupting normal fermentation can lead to rapid changes in the pH (acidity) of the intestine, which further impact the normal population of bacteria and the digestive process. There are many species of bacteria living within the colon, including ones that can cause illness if their population numbers become too high. It is basically a situation where large numbers of the so-called "good" bacteria actually keep the numbers of the potentially "bad" bacteria under control (competitive inhibition). This is one reason why the administration of some oral antibiotics can cause diarrhea. This is also why it is extremely important to make any changes in type and quantity of feed slowly.
It is generally a good idea to bring your own feed to shows so you do not have to make a sudden change in feed during times of other stress. Even with hay, it is probably beneficial to feed half of the "old" batch and half of the "new" batch for about a week before completely making the change. This also holds true for fresh, lush grass in the spring--it is best to introduce horses a little bit longer each day if they are going to be turned out for long periods of time.
Almost every winter I care for a horse experiencing colic from suddenly receiving too much soluble (fermentable) carbohydrate (like cracked corn). In those cases, the grain ration is increased quickly under the mistaken idea that this will give the horse more readily available calories to help him keep warm during the really cold season. This is not only dangerous, it is incorrect!
Sudden increases of the more easily fermentable feeds (the concentrated grain products) cause the fermentation rate to dramatically increase and certain species of bacteria to proliferate. This increase selects out a species of bacteria that produces many acid byproducts, in turn dropping the pH within the large intestine (making it more acidic) and subsequently killing off some of the "good" bacterial population. This causes a severe disruption in normal fermentation and gastrointestinal motility.
Some of these horses actually die from the stomach rupture because of the pooling of excessive fluid and gas since the horse cannot vomit. Others can develop severe diarrhea and/or laminitis (founder). So-called grain overload can cause very serious illness, so it is important to keep your grain products in a place where a loose horse can't get at them, and never start feeding large quantities of a grain product suddenly.
A significant percentage of body heat in the horse can come from the normal slow fermentation of fiber within the large colon. So to help your horse keep "warm" in the winter, make sure he has access to good-quality hay. For those who live in a climate where the winter is cold (below freezing), the caloric requirement in winter can increase 25% or more (depending on how cold it gets).
Does Size Matter?
Another factor that affects the fermentation process is the particle size of the feed material traveling through the colon. If the particle size is too large (as in an older horse with bad teeth that can't chew properly), it will not be fully digested/fermented in the intestines. Finding two- to three-inch long stems of under-digested hay in the manure can be a clue to this.
On the flip side, if the particle size is small, it is much more quickly fermented because more bacteria have access to more surface area of that amount of feed material. This is why cracked/ground corn can cause problems. In some cases, feedstuff with very small particles can actually move through the intestinal system too quickly, thus not allowing enough time for it to be fully processed by the small intestine.
Equine nutrition has expanded by leaps and bounds in the last decade. Nutritional imbalances have been proven to be linked to such diseases as osteochondritis dissecans (a bone maturation disorder), and nutritional therapy has proven beneficial in tying-up and glycogen storage disease. The types of processed feeds and supplements available are growing exponentially, and your choices for an individual horse should be educated ones.
There are a number of excellent books available, and one of the best general references is the National Research Council's Nutrient Requirements of Horses published by the National Academy Press, Washington, D.C., in 1989 (available online at www.nap.edu/books/0309039894/html). This book is currently being revised and the new edition should be available in the next year or so. Another good reference is Lon Lewis' Feeding and Care of the Horse (second edition).
In addition to your veterinarian, many state colleges have an agriculture or equine extension agent who can be of great help with ration balancing and forage/feed analysis. There is also a growing number of private equine nutritional consultants.
However you choose to create a ration for your horse, make sure it is one that suits the horse's age, activity level, nutritional requirement, and climate. It is said that you are what you eat, and the same is true of your horse. Except that you as owner are responsible for choosing what your horse eats every day.
About the Author
Michael A. Ball, DVM, completed an internship in medicine and surgery and an internship in anesthesia at the University of Georgia in 1994, a residency in internal medicine, and graduate work in pharmacology at Cornell University in 1997, and was on staff at Cornell before starting Early Winter Equine Medicine & Surgery located in Ithaca, N.Y. He is also an FEI veterinarian and works internationally with the United States Equestrian Team.
Ball authored Understanding The Equine Eye, Understanding Basic Horse Care, and Understanding Equine First Aid, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.