Often, it is the way rations are fed rather than their composition that leads to digestive upset in horses. Even under the best of management, several anatomical peculiarities of the horse’s digestive tract predispose horses to digestive disorders such as colic and founder. Under poor feeding management, the onset of these disorders is almost assured. The objective of feeding management is to provide a ration with balanced nutrition in a manner which maximizes nutrient utilization while lessening the occurrence of digestive disorders.
Anatomical Peculiarities of the Equine Digestive Tract
The horse’s digestive tract can be divided into two functional divisions: foregut and hindgut. The foregut of the horse is made up of the mouth, esophagus, stomach and small intestine. It functions similarly to the digestive tract of the pig in that it is made of a simple, one-compartment stomach, followed by the small intestine. The hindgut of the horse is comprised of the cecum, large colon, small colon and rectum. The cecum functions much like the rumen of a cow in that it is a relatively large, fermentative vat housing microbes which aid digestion. These microbes break down nutrient sources that would otherwise be unavailable to the horse. Each part of the digestive tract has peculiarities that relate to feeding management.
The mouth is responsible for the initial breakdown and swallowing of feedstuffs. Chewing reduces the size of large particle feedstuffs and breaks up the less digestible, outer coverings of grains and forages. Additionally, mastication stimulates salivary glands to release saliva, which assists in lubrication of feed for swallowing.
Since proper denture conformation is necessary for mastication, inspection of the horses’s teeth by a qualified individual should be a routine management procedure. As horses age, dental conformation can be expected to deteriorate. Consequently, older horses require more frequent inspection and treatment of teeth. Signs of poor dental conformation include excessive loss of feed while eating, positioning the jaw or head sideways while chewing and evidence of general loss of condition and thriftiness.
The diameter and tone of the musculature of the esophagus make it difficult for the horse to expel gas through belching or vomiting. These are predisposing features to gastric rupture, gastric distention and colic.
Compared to most livestock, the size of the horse’s stomach is small, about 10 percent of the volume of the total digestive tract. The small size makes the rate of flow of feed material in the digestive tract through the stomach relatively fast. Gastric emptying is dependent upon volume, so large meals can be expected to pass more quickly than feed eaten continuously at low volumes. Studies have shown the majority of feed material in the digestive tract passes to the small intestine within 12 hours following a meal.
The small intestine is the main site of digestion and absorption of protein, energy, vitamins and minerals. Similar to the stomach, intake level of the feed influences rate of flow of ingested matter through the small intestine. Large amounts fed in meal feedings increase rate of flow to the large intestine.
Cecum and Colon
Ingested matter not previously digested or absorbed in the small intestine flows to the cecum and colon, which make up about 50 percent of the volume of the digestive tract. The cecum and colon house bacterial, protozoal and fungal populations which function in microbial digestion of feed material in the digestive tract. Many different products of microbial digestion are absorbed by the horse.
Passage of ingested matter through the large and small colon is relatively slow. Rates of flow through the colon may take up to several days following the time feed was eaten. The diameter of different segments of the large colon varies abruptly. Additionally, the arrangement includes several flexures where the colon turns back onto itself. Anatomical arrangements such as these predispose the horse to digestive upset when nutrient flow is abnormal.
Nutrient Intake and Digestion
The daily minimal requirement for water has been estimated to vary from 5 to 20 gallons. Requirements depend on factors such as environmental temperature, workload, production state and intake. Voluntary water intake can be expected to increase as the amount of ration eaten increases. Also, rations low in digestibility increase water intake. Furthermore, horses can be expected to drink more frequently when exposed to hot environmental temperatures. Horses exercising in temperate environments may have increases of 300 to 400 percent in water requirements for replacement of water that is loss in expired air and sweat. Since restriction of water intake may cause digestive upset, recommendations generally are for free choice access to clean, palatable water.
Energy is the fuel for chemical reactions which run the various systems of the body. Energy-containing compounds are part of grains, forages and many supplements. Energy is supplied in the form of starch, fiber and fat.
Starch is found mainly in grains, and as much as 55 to 85 percent of starch is absorbed in the small intestine. Starch bypassing to the hindgut is digested by microbes and absorbed as volatile fatty acids. Large amounts of starch presented to the hindgut predispose horses to colic because of gaseous products of microbial digestion and abnormal changes in gut pH and fluid balance. The amount of starch bypassing to the hindgut depends on intake level, rate of flow through the digestive tract and amount of mechanical disruption of the hard seed coats of grains. Results from nutritional studies suggest that approximately two grams starch per pound of a horse’s body weight increases starch bypass to the point of causing digestive upset. Considering starch levels in typically formulated grain mixes, recommendations are to split daily grain needs to two or more daily feedings when grain levels are greater than 0.5 percent of body weight per day (example:5 to 6 pounds of grain for a 1,000-pound horse).
Hay and pasture forage are the most common sources of high-fiber feeds fed to horses. Fiber digestion is dependent on the efficiency of digestion from microbial fermentation in the cecum and colon. Compared to cattle, horses are less efficient in digesting most sources of fiber, presumably because of faster rates of passage of ingested matter. Also, fiber digestion is dependent on the maturity and type of forage. Mature, stemmy forages are inefficiently digested, whereas digestion of immature, leafy, small-stemmed sources of fiber are similar in horses and cattle. Processing hays in cubes, pellets or chop has little effect on digestibility but may be helpful for feeding to older horses with poor teeth condition.
Fat is a component of most feedstuffs. Non-supplemented grain mixes typically have minimums of 2 to 3 percent fat. Adding additional levels of fat in formulations for grain mixes has become a common practice. This supplementation increases the energy concentration of grain mixes while decreasing the amount of starch. Therefore, fat-added feeds have advantages of being more concentrated in energy and safer because of containing less starch as a total part of the energy-containing compounds.
Proteins supply amino acids. Amino acids are used in a variety of body processes, largely for developing and maintaining lean body tissue. Amino acids are absorbed intact in the small intestine, while protein in the hindgut is absorbed primarily as ammonia. Some of the essential amino acids must be absorbed intact because the horse’s body cannot synthesize them. Thus, increasing the efficiency of protein digestion in the foregut is desirable. Total tract and prececal digestibility vary with protein source and protein concentration in the diet. Total tract protein digestibility of feeds typically ranges from 40 to 70 percent. As much as 75 percent of protein in soybean meal is digested in the foregut, whereas estimates for prececal forage digestibility range from near zero to 20 percent. Slowing the passage of protein by splitting daily needs into two or three feedings per day will increase amino acid absorption in growing horses.
Minerals and Vitamins
Mineral and vitamin imbalances, deficiencies and toxicity can cause a multitude of health disorders in the horse. In many cases, recommendations are based on limited research or requirements and have not been established because of absence of research.
Calcium and phosphorus are the two minerals which have received the most research attention. Horses require more calcium than phosphorus and are susceptible to skeletal system disorders when fed less calcium than phosphorus. Additional minerals receiving considerable attention in recent research include copper and zinc, also because of implications related to skeletal growth disorders.
Research information on vitamin requirements is largely absent in equine nutrition. Fresh forage is a major source of vitamins, and most needs are considered met when horses have access to quality hay or pasture. Vitamin A is the most commonly supplemented vitamin in rations, partially because of the large needs for production and growth. Vitamin D is also routinely added, especially to horses who do not receive fresh forage. The needs for vitamin D are less than for vitamin A, and recommended upper levels of safe intake are much lower.
Requirements of the other fat-soluble vitamins, E and K, are less clear, and clinical deficiencies and toxicity are not as commonly observed. Sources of vitamin E are routinely added to equine diets to guard against deficiencies which cause myodegeneration, or breakdown of muscle. Vitamin K requirements are presumed to be met by synthesis of vitamin K sources by microbes in the cecum and colon. Requirements for B vitamins are largely unknown. B vitamins are assimilated by microbes in the horse’s cecum and colon, and these sources are assumed to meet the needs of most horses. However, B vitamin supplements are routinely added to diets of exercising horses because of the role of B vitamins as catalysts for energetic pathways.
Feeding Management Implications
As discussed previously, water intake is important for maintenance of normal body processes. Restrictions in water, such as that caused by voluntary reductions in response to abrupt decreases in environmental temperature or changes in water source, may cause an increase in the incidence of colic. Water intake should be monitored because of numerous health problems associated with dehydration. Monitoring water intake requires frequent inspection of water sources, including the function of automatic waterers. In general, horses should be allowed free access to fresh, palatable water. Some horses may drink so much as to cause digestive upset if given free access immediately prior to performing and recovering from intense exercise, and it is recommended that water should be provided in smaller amounts at frequent intervals during these times. Regardless, dehydration can be a serious problem in exercising horses. Therefore, it is important that water is offered frequently and that intake is monitored.
The Need for Long-Stem Forage
Rations for horses should be forage based. Generally, horses should have access to pastures, hays or coarsely processed forage at minimal levels of 0.75 percent of body weight per day. Among other benefits, incorporating long-stem forage into rations increases particle size of ingested matter, thus slowing rate of passage. It also increases dry matter intake, thus stimulating water intake. Additionally, incorporating long-stem forage reduces the frequency of behavioral problems such as tail chewing, wood chewing and feeding on excrement. Grain mixes should be formulated to balance and add to the value of forages. High quality forages are more concentrated in nutrients and more efficiently digested; thus, lower levels of grain supplementation are necessary. Feeding forages containing weeds, insects, large amounts of indigestible fiber or foreign material will predispose the horse to digestive upset.
Meeting Requirements with Balanced Rations
Nutrient balances are important for all diets. However, horses in production, growth or performing high levels of athletic competition or work are most likely to develop observable disorders from ingesting an imbalanced ration. A feeding management plan requires knowledge of requirements, an ability to formulate rations and knowledge of utilization of different feedstuffs.
Growing horses, exercising horses, gestating and lactating mares and stallions during breeding programs require more nutrients than horses at maintenance. Feeding management plans should consider these differences, and farm facilities should separate horses into different production classes. The feeding management plan should also consider the number of different classes of horses, the ability to correctly add supplements on-farm, the ability to feed different numbers, as well as types of rations, feeding costs and the availability of different feedstuffs.
Meeting requirements also requires knowledge of nutrient content of grains, forages and supplements. Rations have successfully incorporated many different combinations of fresh forage, hays, grains and supplements. However, feedstuffs contain different levels of energy, protein, minerals and vitamins. Knowing the expected nutrient profiles of selected feedstuffs will direct supplementation to meet needs without causing deleterious effects on performance or health.
Estimates of the nutrient content of feedstuffs can be obtained from feed tags on grain mixes, feedstuff tables in animal nutrition texts, professional nutritionists or chemical analyses. Because of the variability in forages, farms using significant amounts of hay from a single source should have hay sources routinely tested for protein, fiber and minerals.
Maintaining a Nutrient Balance in Rations
Feedstuffs contain differing levels of nutrients. Grains are relatively higher in energy than forages, some byproduct feeds contain high levels of protein, and mineral and vitamin levels can be expected to vary greatly between different feed sources. Because of these differences, changing sources or amounts of feedstuffs will alter the nutrient balance in rations. Commercially formulated grain mixes are routinely supplemented with nutrients, so the different ratios of grain and hay and different hays that horses are fed will not adversely alter the nutrient profile of the total ration.
Some feeding managers are equipped to properly supplement rations by on-farm addition of ration ingredients, whereas others routinely make uninformed decisions to add many different types of nutrients to the base rations. The unknowledgeable addition of ingredients can easily cause numerous irreversible health problems in all classes of horses. Two problems frequently observed with improper ration adjustments are supplementation without knowledge of need or level of intake before supplementation, and supplementing for one ingredient without recognizing the additional amount of other ingredients a supplement may contain.
Additionally, horses should not be expected to self-regulate their need for most nutrients. This is evidenced by horses overconsuming energy to the point of digestive upset. In addition, horses do not regulate most of their mineral needs under free-choice management. Additional needs for minerals should be met as part of a formulated ration at regulated intakes. The exception to this rule is the free choice offering of salt, or sodium chloride. It is generally recommended that all classes of horses be provided salt, either plain or trace-mineralized, in block form with the constraint that free-choice, palatable water is available at all times.
Feeding to Desired Body Condition
Horses in a positive energy balance will store energy as fat, and body fat is reduced when the ration does not provide sufficient nutrients to maintain energy balance. Accurately assessing the fat cover allows for visual appraisal of the energy status of a horse. In general, most horses should be fed a balanced ration at levels which produce a moderate to fleshy body condition, thus avoiding an extremely thin or obese condition. Because horses in similar production and weight classes will vary in their nutrient needs, routine assessment of body condition of each horse is necessary. While horses in similar production and weight classes are commonly grouped together, those individuals with abnormally high or low body condition may need to be separated further to meet individual needs.
Assessing Energy Sources, Levels and Utilization in Feeds
Voluntary intake in horses appears to be influenced by a number of factors: weather, palatability of feed, interaction with other horses, and energy intake, among others. Regardless, if allowed free access, most horses will consume enough grain to cause digestive upset. As discussed previously, the most common problem with overeating is the consumption of too much starch in a single feeding. Grains vary in the amount of starch. For example, corn has more starch per pound than oats. Also, there may be differences in foregut digestibility of starch between different grains. Depending on intake, more starch in oats may be digested prececally than corn.
The Value of Processing Feeds
Processing increases digestibility of hard seed-coat grains and assists in intake of ingredients with different particle sizes in a mix. Feeding finely processed rations such as ground mixes is not recommended because it may decrease palatability, increase dust, increase incidence of gastric upset and increase the rate of flow of nutrients through the digestive tract.
Pelleting, micronizing, flaking, rolling, cracking, wafering and extruding are examples of processing methods that are acceptable. Several different pellet sizes have been successfully fed to horses, most ranging from 0.2 to 0.75 inches in diameter. Often, forages are recommended to be fed loose so behavioral abnormalities resulting from boredom are reduced. However, cubed (1 1/4 inch in diameter) hay can be fed as the sole source of forage with no reported incidence of behavior abnormalities.
Processing can cause several differences in rate of intake and utilization of nutrients. Completely pelleted rations are consumed faster than textured grains. Extruded feeds are consumed more slowly than pelleted or textured grain mixes. Texture and hardness of grains will determine the value of processing. Small seed grains with hard seed coats, such as milo and wheat, should be processed to increase utilization of nutrients. The benefit of processing softer seed-coated grains, such as oats, is much less. Also, the value of processing grains can differ between horses. Horses with poor denture conformation, such as older horses, may benefit more from processed feeds than others. Also, the value of processing is increased when feeding large quantities of grain to horses with limited capacity, such as rations fed growing horses to obtain maximum gain.
Total rations may be mixed, ground, and processed by pelleting or extrusion to make a complete feed. Complete feeds have several advantages, most related to ease and convenience of feeding. However, it is most commonly recommended to provide at least 0.75 pounds per 100 pounds of body weight in long-stem forage to supplement these complete feeds to guard against tail chewing, coprophagy and gastrointestinal problems.
Feeding by Weight of Ration Instead of Volume
Feeding by weight will decrease the chance of overfeeding due to differences in weight per volume of different feeds and different processing methods. For example, corn weighs more per volume than oats, and pelleted feeds weigh more per volume than textured feeds. Consequently, it is recommended to weigh feed periodically to insure accurate monitoring of intake. This is especially important when changing feed sources. One of the most common causes of digestive upset is overfeeding energy in a single feeding because differences in weight of grain mixes were not taken into account.
In many ways, the horse’s digestive physiology is best suited for a continuous, low-level supply of feed. However, for management, housing and production needs, most horses are meal-fed. Meal-feeding large amounts of starch increases starch bypass into the cecum and colon. As discussed previously, large amounts of starch presented to the hindgut increases the frequency of digestive upset. Therefore, it is recommended to split grain into two daily feedings when the daily amount of grain exceeds 0.5 percent body weight (5 pounds grain per 1,000-pound horse). Those feeding grain to horses at levels of or above 1 percent of body weight per day should consider splitting amounts into three portions per day. Meal feedings should be separated as much as possible — that is,10 to 12 hours between a.m. and p.m. feedings for two daily meals.
Reducing Rate of Intake
Reducing rate of feed intake may be desirable if horses bolt their feed, resulting in choking or digestive upset, or if reducing rate of intake decreases competition in group-fed horses. When horses are fed in individual feeders, methods used to slow feed intake in abnormally fast eating horses have included spreading grain out in shallow troughs, placing several large stones in the feed trough, requiring the horse to eat around them or using spaced bars or feeding rings to limit access to the feed trough. As discussed previously, processing of the ration also influences the rate of intake. While the fiber content or size of pellet does not seem to affect rate of intake, increasing pellet density, or hardness, has been shown to slow intake of a pelleted grain mix.
Group versus Individual Feeding
In groups, horses tend to do what other horses do. One horse eating encourages others to eat. Similarly, appetite can be stimulated in individually housed horses by allowing a horse to observe other horses eating.
Competition among horses in group-fed situations may allow some horses to consume more feed than needed while others are not allowed access to adequate amounts. To reduce competition among horses, group-housed horses should be fed grain in individual feeders that are spread out over a large area, that is, 50 feet between feeders. Additionally, slowing the rate of intake of grain by reducing the desire to eat may reduce competition. Supplementing pastures with free choice hay in times of limited forage production may slow rate of intake of grain because horses may not be as hungry at meal time.
However, even under the best management, horses low on the herd pecking order or stressed because of conditions such as old age or lameness will need to be housed separately to reduce competition.
The Need to Make Gradual Changes in Rations
Grains and hay differ in nutrient content. Changes in the intake level and the physical form of rations should be done gradually over several days to weeks. This practice allows the digestive tract time to adapt to different levels and physical forms of nutrients and is especially important when feeding energy-dense rations. As such, grain amounts should be increased incrementally when changes in management require an immediate need for more energy. For example, increase grain one-half pound every two to three days until energy balance is met. For similar reasons, introduce horses to pastures with large amounts of lush forages by limiting access for several days.
Incorporating the Feeding Management Plan with Total Farm Management
The source, ingredient mix and number of rations will depend on numerous management practices that interrelate with the feeding program. The need to transport to events, timing of exercise schedules, labor constraints and costs are significant management factors which affect feeding management. Deworming, vaccination schedules, ectoparasite control and general hygiene are examples of health practices that relate the nutritional plans and the well-being of the horse.
Effective management also involves treating each horse as an individual. As such, effective management requires an accurate, quantitative record-keeping system that allows for individual assessment of each horse.
David W. Freeman, Retired Extension Equine Specialist