Feeding horses based on their gastrointestinal (GI) tract design and biological needs can help improve their gut and overall health. Their small stomach capacity is best suited for grazing and consuming small meals throughout the day.

“A good feeding program will provide adequate energy and feed for (a horse’s) gut health by providing long-stem fiber needed to ensure the gastrointestinal tract is moving well and consistently,” said Robert Bray, PhD, professor emeritus at California State Polytechnic University, in Pomona, and consulting equine nutritionist for Star Milling Company, in Perris, California, during the 2024 EquiSummit virtual conference, held Dec. 3-4.

To feed horses by gut design, Bray said owners should follow two important rules for feeding fiber: the 1.5% rule and the 50% rule.

What is Fiber?

“Fiber is the horse’s primary fuel source that is digested by microbes in the large intestine,” said Bray. “Fiber promotes GI tract movement and when the horses chew, it produces saliva that buffers the foregut (everything ahead of the cecum). Fiber nourishes the gut microbes, promoting microbial diversity and stabilizing the pH of the GI tract. “Fiber also establishes the rate food moves through the GI tract.”

Bray described fiber as the cell wall component of a plant cell. In contrast, the contents of the cell include the starches, proteins, sugars, trace minerals, etc.

He emphasized that fiber is not considered a nutrient, yet it is an essential component of the diet, often noted on feed tags as “crude fiber.”

The 1.5% Rule for Feeding Horses FiberHorses need to eat at least 1.5% of body weight per day in fiber. So, an average 1,000-pound horse needs 15 pounds of fiber.

Fiber can come from a variety of sources and be found in different amounts depending on the type of forage. For example, 15 pounds of timothy hay provides 4.5 pounds of crude fiber, or 2,043 grams per day (based on 30% crude fiber).

“Other grass hays have similar levels of crude fiber such as teff, Bermuda, orchardgrass, and oat hay,” said Bray. “But let’s look at alfalfa hay that owners commonly feed.”

Fifteen pounds of a reasonable-quality alfalfa offers only 1,736 g of crude fiber per day, which is much lower than the crude fiber in grass hays noted above. “This alfalfa example will only fill 86% of the horse’s fiber requirement,” said Bray.

The 50% Rule of Feeding Horses Fiber

Because alfalfa hay does not fulfill a horse’s fiber requirement, it should not exceed 50% of the total forage because pound per pound it contains less fiber than grass hay.

“In addition, horses will often select the alfalfa leaves instead of the stems, so they are getting even less fiber,” said Bray. “Even though alfalfahas less fiber, it has higher energy (more calories) than the grass hays so, to maintain body weight, they (horse owners) need to feed less, which means they (horses) get even less fiber.”

If we split alfalfa 50/50 with timothy hay, we are still only feeding 94% of the horse’s total fiber requirement, he explained, but it’s better than alfalfa alone.

“If you feed a more mature grass hay, then the fiber content will increase even more,” said Bray.

Regardless of the type of hay your horse eats, feeding it in a slow feeder might improve GI tract motility, stimulate saliva production to buffer gastric juices, and decrease time between feedings.

Looking at Processed Forages and Complete Feeds

Just like long-stem alfalfa, horses should not consume more than 50% of their dietary fiber in the form of alfalfa cubes or pellets. Processed forages and hay offer the same level of digestibility.

“Cubes have less volume which induces less GI tract contraction/gut motility and stimulates less water consumption,” said Bray. “This is a negative. In addition, feeding intervals are longer because they consume the cubes and pellets quicker, leaving them with an empty gut.”

Bray, who admits he’s not a fan of complete feeds—designed to meet all of a horse’s nutritional needs, including forage—explained, “If you look at fiber requirements, 15 pounds of a complete feed only fulfills 81% of the horse’s fiber requirements. This is far off from the target.” To meet those requirements, you’ll need to supplement with additional forage.

Other Fibers for Horses

Beet pulp and soy hulls are also referred to as super fibers, but Bray prefers the term soluble fibers. These fibers contain pectins and oligosaccharides (short carbohydrate chains), which horses digest in the small intestine unlike regular fibers that are digested in the large intestine.

“One pound of soy hulls or beet pulp can provide a significant amount of fiber,” said Bray. “Still, it is important to provide adequate long-stem forage to promote GI tract motility.” Beet pulp is typically soaked and soy hulls softened with water.

Take-Home Message

Long-stem grass forages fulfill a horse’s daily fiber needs when fed at 1.5% of his body weight per day. These promote GI tract motility, stimulate saliva production to buffer gastric juices, and decrease the gap between feedings, especially if fed in a slow feeder.

Additional 2024 EquiSUMMIT Coverage:

• Tackling Equine Gastric Ulcers
• Investigating Prebiotics’ Role in Managing Equine Leaky Gut
• Benefits of Extruded Horse Feeds
• What Happens With the Horse’s Gut in the Heat?
• The Equine Microbiome and Immunity

Gastric ulcers are sores in the equine stomach lining that cause distress and negatively affect performance. An estimated 60% of performance horses and 90% of racehorses have gastric ulcers. The odds of a horse developing gastric ulcers increase with stress such as exercise or life changes.

“Such stresses include training, trailering, and showing … all things we do with our horses all the time, putting our horses at risk,” said Jyme Nicols, PhD, director of nutrition for Bluebonnet and creator of the Feed Room Chemist Podcast during her presentation at the 2024 virtual EquiSummit, held Dec. 3-4.

Where Do Equine Gastric Ulcers Occur?

Horses have one very small (3- to 5-gallon) single-chambered stomach, which functions best when they are allowed to eat small amounts of food all day. The stomach is made up of two key areas: an upper area covered by squamous tissue and a lower covered by glandular tissue. The lower glandular region produces acidic gastric juice and mucus.

“It is very important to know where ulcers are—the squamous, glandular, or both—because location will influence treatment protocol,” said Nichols. “And the only way to diagnose ulcers is to have the horse ‘scoped’ by a veterinarian,” meaning with the use of a gastroscope.

How to Score Equine Gastric Ulcers

In 1999 researchers introduced the term equine gastric ulcer syndrome (EGUS) as an umbrella term for any ulcer in the horse’s stomach. Experts later reported that ulcers in the glandular and squamous region are very different, and the two types should be described independent of one another.

For squamous ulcers, in the upper region of the stomach, veterinarians use a well-established grading system, which describes ulcers based on severity. The scale ranges from 0 (healthy with no ulcers) to 4 (extensive lesions, deep erosions). In contrast, practitioners have no universal scoring system for glandular ulcers (in the lower region of the stomach). Instead, they describe those ulcers based on their physical appearance and specific location (e.g., nodular, flat, raised, depressed, and in the antrum, pylorus, etc.).

Causes and Risk Factors for Equine Gastric Ulcers

“As mentioned above, we ask horses to do things they aren’t designed to do such as training, trailering, and showing,” said Nichols. “They are designed to have their heads down, grazing, in a herd, yet we isolate them in stalls, travel, and hire trainers to work with them. We often feed them meals only twice a day, sometimes with large amounts of grain, and many owners don’t have the luxury of turning their horse out all day.”

Some key risks factors for developing EGUS include:

• Extended periods without forage. Going six hours or more between forage meals (hay or pasture) increases a horse’s risk, and many horses are going beyond six hours, said Nichols. Extending feeding time and ensuring horses have forage available at all times can help decrease the risk.
• Insufficient water. Water serves as an excellent buffer for gastric juices, and horses that drink well and always have access to water at all times are less likely to develop gastric ulcers. In contrast, horses with restricted water access are more than two times more likely to have ulcers.
• Consuming high-starch meals exceeding 2 g/kg of bodyweight doubles the horse’s risk of developing ulcers.
• Medications such as non-steroidal anti-inflammatory drugs (NSAIDS, e.g., phenylbutazone and flunixin meglumine) increase the risk of glandular disease especially, by damaging the gastric mucosal lining.

Clinical Signs of Equine Gastric Ulcers

 “Signs of EGUS can be subtle, so you have to know your horse and what is normal in order to recognize what is abnormal,” said Nichols.

Subtle behavior changes, such as changes in facial expression when saddling (ear, eye, and mouth position and movements), can suggest the presence of gastric ulcers. Other signs of EGUS can include general irritability and girthiness (e.g., the horse might try to bite cross-ties or you, swish his tail) and changes in performance.

Certain indicators of ulcers can become more apparent, such as dropping condition, losing topline, dull haircoat, and exhibiting signs of colic.

Diagnosing and Treating Equine Gastric Ulcers

Always have your veterinarian perform a gastroscopy on your horse if you think he might have gastric ulcers.

“Be aware that there is no link between glandular and squamous disease,” said Nichols. “Having one does not mean they do or do not have the other. Plus, there is no link between outward signs and the severity of ulcers.”

Treating ulcers mainly involves suppressing acidity in the stomach and protecting the gastric mucosal lining. The most commonly prescribed medications include omeprazole, ranitidine, and sucralfate.

“We also need to consider what led to ulcers in the first place,” said Nichols. “Align their daily lives with how they are physiologically designed—let your horse be a horse.” Management changes such as increasing forage available, reducing stressors in the environment, and limiting changes in routine can help decrease your horse’s risk of developing gastric ulcers.    

Take-Home Message

“Be aware that most horses are at risk of developing gastric ulcers, so pay attention to your horse’s behavior to appreciate early signs,” Nichols said.

Prevent gastric ulcers whenever possible by managing your horse’s diet, the environment, and his stress levels, and work closely with your veterinarian and equine nutritionist to diagnose, treat, and manage EGUS.

Additional 2024 EquiSUMMIT Coverage:

• Rules for Feeding Horses Forage
• Investigating Prebiotics’ Role in Managing Equine Leaky Gut
• Benefits of Extruded Horse Feeds
• What Happens With the Horse’s Gut in the Heat?
• The Equine Microbiome and Immunity

Leaky gut in horses has rapidly become a widely discussed topic, yet most of what veterinarians know about this condition has been extrapolated from other species. During her presentation at the virtual 2024 EquiSummit, held Dec. 3-4, Wendy Pearson, PhD, associate professor of equine physiology in the department of animal biosciences at the University of Guelph, in Canada, shared results from her research team’s recent study, showing leaky gut might be manageable with prebiotics.

Defining Leaky Gut in Horses

The lining of a horse’s gastrointestinal (GI) tract is a single layer of cells separating the lumen of the GI tract (where food is stored) from the horse’s bloodstream. In a healthy GI tract, cells on the lumen side engulf the nutrients and release them on the bloodstream side. Some small molecules can squeeze between the cells to reach the bloodstream.

“The tight junctions between the intestinal cells stop larger molecules like bacterial products such as lipopolysaccharide (LPS),” said Pearson. “Small molecules and solutes such as sodium ions can easily pass though the tight junctions.”

But when the cells lose those tight junctions, a leak pathway forms between the cells, allowing larger molecules, such as the bacterial products and toxins, to cross. Toxins then enter the circulation and can contribute to the development of low-grade inflammation. In other species this low-grade systemic inflammation has several consequences; it impairs skeletal muscle metabolism, affects metabolic function, and contributes to allergies or chronic inflammatory disorders such as arthritis, among other effects. In theory, this can also occur in horses, but no research exists to support this hypothesis.

Factors Contributing to Equine Leaky Gut

Researchers conducting work in other species have identified a few contributors to leaky gut, including:

• Individual bouts of exercise. “When animals exercise, blood is diverted away from the GI tract and toward exercising muscles,” said Pearson. “As a result, the decreased blood flow to the intestinal cells causes damage, swelling, and inflammation, making them leaky.”
• Stressful situations. Cortisol compromises the tight junctions and disturbs the intestinal microbiome—the microbes that reside in the GI tract, playing important roles in digestion, immunity, and nervous system function. Stressful situations could include transport and heat stress.
• Diet. This can either positively or negatively affect leaky gut, depending on the diet composition and its effect on the microbiome.

Leaky Gut Research in Horses

In 2023 Pearson and her lead graduate student, Melissa McGilloway, published two studies designed to develop a model for studying leaky gut in horses. They chose trailering and exercise for the model because those activities mimic a typical day for many horses.

They transported 16 horses for one hour then immediately exercised them for 30 minutes. Before trailering they gave the horses oral iohexol (a contrast agent).

“Iohexol is a small molecule that does not fit through a healthy tight junction but will pass through the leak pathway in horses with leaky gut,” said Pearson. If the horses have leaky gut, the iohexol will pass from the lumen of the GI tract, through the leak pathway between the intestinal cells, and enter the bloodstream, where the researchers would be able to measure it.

They collected blood samples immediately before administering iohexol, after a one-hour trailer ride, after 30 minutes of exercise, then again intermittently for eight hours after exercise. The horses in the control group stayed at the barn. The researchers administered iohexol and took blood samples at the same time as they did for horses undergoing transport and exercise.

“We found that the blood levels of iohexol were significantly increased in the stressed horses for up to two hours after exercise, showing that leaky gut did develop,” said Pearson.

Can We Prevent Leaky Gut in Horses?

Based on research in humans, Pearson’s team tested a fermentation product of the mushroom, Aspergillus oryzae, designed to support the GI microbiome and limits stress-induced GI tract hyperpermeability.

They fed the horses this prebiotic for 28 days and then repeated the study.

“The horses that were transported and exercised had no spike in iohexhol, indicating that the leak pathway did not form in horses fed the fermentation product of A. oryzae,” Pearson said.

Take-Home Message

Pearson’s team has shown that normal, daily events such as transport and exercise can induce leaky gut in horses, but feeding the prebiotic A. oryzae blocked that hyperpermeability in this study group. More research is needed to fully understand the uses of this prebiotic.

Additional 2024 EquiSUMMIT Coverage:

• Tackling Equine Gastric Ulcers
• Rules for Feeding Horses Forage
• Benefits of Extruded Horse Feeds
• What Happens With the Horse’s Gut in the Heat?
• The Equine Microbiome and Immunity

Over the years, horse feeds that were once mixes of whole grains have evolved to textured, pelleted, and extruded forms to meet equine nutritional needs. Feed form impacts a horse’s overall health, with extruded feed designed to be more digestible.

Randel Raub, PhD, director of research and nutrition for the Kent Nutrition Group, headquartered in Muscatine, Iowa, said he believes extruded feeds are a superior form of horse feed. “My message is not subtle because I have a firm conviction that extruded feeds differentiate themselves from pelleted and textured feeds,” Raub said during his presentation at the 2024 EquiSummit, held virtually Dec. 3-4.

What Are Extruded Horse Feeds?

Mills produce extruded feeds by finely grinding ingredients used in horse feeds, then adding moisture, heat, and pressure to essentially cook the ingredients. This creates a doughlike mixture that is forced through an extruder to produce pieces in an array of shapes, sizes, and densities depending on the die used in the extruder.

Grinding the ingredients effectively increases the surface area of the feed, making it more digestible, he explained. The cooking process facilitates a pre-digestion process.

“For example, starch and sugar are gelatinized, which make them more easily digested in the small intestine,” said Raub. “In addition, proteins are broken down into individual amino acids, which typically occurs in the horse’s small intestine.”

Manufacturers apply ingredients sensitive to temperature and pressure (such as some vitamins and other specialized ingredients) post-extrusion. This ensures the horse receives a balanced diet, and the final product meets the nutrient guarantees on the tag or label.

Benefits of Extruded Feeds

Extruded feed is puffier, bulkier, and less dense than pelleted feed, giving it a greater volume. “Being a larger volume, the horse is going to take longer to consume that feed, and getting horses to slow down the consumption of the concentrate portion of the diet is good,” Raub explained.

Extruded feed also requires 50% less pressure (pounds per square inch) for horses to chew, making it easier to break down and form a food bolus that can be swallowed. This can be particularly important in senior horses with dentition problems.

“Many pellets in a pelleted feed are not thoroughly crushed by the horses’ molars,” said Raub, which can lead to choking. In contrast, extruded feeds quickly turn into a mash when mixed with saliva, allowing horses to swallow easily and the esophagus to move it smoothly to the stomach.

In addition to reducing choke, extruded feeds:

• Reduce pathogens that could be inherent in some feed ingredients because of the heating process.
• Have longer consumption times per pound of feed, which might be beneficial in reducing the incidence and severity of gastric ulcers. “More chewing produces more saliva, which is a natural buffer for the acidic environment in the stomach,” Raub said.
• Improve ingredient digestibility because the fine grind and precooking predigests the feed. Improved pre-cecal (before it reaches the cecum—the first part of the hindgut, which also consists of the large and small colon) digestibility might help decrease cecal upset and colic by ensuring the feed ingredients’ starch and sugar components digest in the small intestine before reaching the hindgut.

Although extruded particles break more easily under pressure, these particles are less prone to creating dust than pellets, making it beneficial for a horse’s respiratory health. “Even though the vast majority of dust in the horse’s environment is from bedding and hay, anytime we can eliminate dust is good,” said Raub.

Take-Home Message

Good nutrition lies not only in the ingredients but also the form in which the horse eats those ingredients. Extruded feeds offer a multitude of health benefits, including easier digestibility, longer consumption times, and reduced dust and pathogens, said Raub.

When transitioning your horse to an extruded feed, proceed slowly over five to seven days. Monitor your horse’s body condition for any changes.

Additional 2024 EquiSUMMIT Coverage:

• Tackling Equine Gastric Ulcers
• Investigating Prebiotics’ Role in Managing Equine Leaky Gut
• Rules for Feeding Horses Forage
• What Happens With the Horse’s Gut in the Heat?
• The Equine Microbiome and Immunity

If your horse sweats and appears outwardly comfortable, he must be managing the high temperatures just fine, right? Not according to Clair Thunes, MS, PhD, equine nutrition consultant and owner of Clarity Equine Nutrition. During her presentation at the 2024 EquiSummit virtual conference, held Dec. 3-4, Thunes said horses suffering heat stress might appear outwardly unaffected. However, their gastrointestinal (GI) tracts struggle, which affects their health and performance.

How Horses Manage Heat

Horses heat up not only from the sun shining on them but also the heat generated during their metabolic processes. For example, the chemical reactions muscle cells use to contract large muscle groups during exercise all produce heat. Horses’ bodies need to manage heat to stay in a thermoneutral zone.

The key way horses lose heat is through evaporation of sweat, said Thunes, “and when that doesn’t work, they rely on respiratory losses through heavy breathing.”

Heat Stress, Injury, and Shock in Horses

Thunes defined heat stress as an increase in body temperature that causes any change in physiology that can impair other body functions. “Heat stress may not cause obvious harm or injury, but the body is nonetheless too hot to do everything it’s supposed to,” she said.

When heat stress progresses to heat injury, noticeable problems start to appear. Horses with heat stroke develop overt clinical signs, including GI tract damage.

Heat-Induced Leaky Gut in Horses

When horses get hot, their bodies reduce blood flow to the GI tract, redirecting it to the large muscle groups to facilitate cooling. This can weaken the gut lining, potentially leading to heat-induced leaky gut—a condition where the cells lining the GI tract lose their tight connections. Instead of maintaining an impermeable layer, gaps develop between the cells, allowing harmful compounds to seep into the intestinal lining and potentially the horse’s bloodstream.

“The presence of endotoxin from bacteria in the intestinal lining and bloodstream initiates an inflammatory response because they are not supposed to be there,” said Thunes. “Degraded food products and digestive enzymes may also leak into the circulation with similar consequences.”

Healthy intestinal tract is lined with villi (fingerlike projections) that absorb nutrients. These can become blunted due to heat stress, resulting in less surface area to absorb nutrients. Horses tend to eat less when heat stressed, which further reduces nutrient availability. Fluid movement in the GI tract can also become impaired, resulting in diarrhea.

In addition to the leaky gut, dysbiosis (microbiome imbalance) occurs in heat-stressed horses. The bacterial population in the intestinal tract can change in diversity, composition, or even the way it functions. “For example, there could be a decrease in the butyrate-producing bacteria, which normally help maintain the GI tract lining,” said Thunes. “With this decrease in butyrate, we see an increase in GI tract inflammation.”

In addition, the digestive tract plays a large role in a horse’s immunity. A leaky gut can trigger the innate immune system, worsening systemic inflammation, and compounds passing out of the gut could result in an allergic reaction.

Supporting Horses in the Heat

In addition to physically cooling horses and minimizing environmental sources of heat, owners can also combat the side effects of heat stress (predominantly leaky gut) by choosing feeds that contain the following ingredients:

• Butyric acid and zinc, which support the cells lining the GI tract. These ingredients will help maintain tight junctions and decrease inflammation; and
• The probiotic Bacillus subtillis PB6, which has been shown to kill endotoxin-releasing pathogens (disease-causing organisms) and help maintain the beneficial population of bacteria in the GI tract.

Take-Home Message

Heat stress negatively affects horses’ gut health, along with their immune system and metabolic function. In addition to reducing sources of heat, both environmental and metabolic, horse owners can provide electrolytes and offer feeds using ingredients known to counter the effects of heat stress, such as butyrate.

“It’s important we’re cautious about working our horses in hot weather and really understand what is happening at a cellular level when horses get hot,” said Thunes.

Additional 2024 EquiSUMMIT Coverage:

• Rules for Feeding Horses Forage
• Tackling Equine Gastric Ulcers
• Investigating Prebiotics’ Role in Managing Equine Leaky Gut
• Benefits of Extruded Horse Feeds
• The Equine Microbiome and Immunity

The equine gut microbiome has been proven to influence many aspects of horses’ health and well-being, including behavior and gastrointestinal health. But why is the microbiome so powerful? Researchers say it’s because of its connections with the nervous and immune systems.

Equine Microbiome Basics

The equine intestinal microbiome refers to the genetic material of the gastrointestinal (GI) microbiota, the diverse population of microorganisms living in the GI tract, said Liz Schatz, DVM, senior specialist of veterinary affairs at SmartPak Equine, headquartered in Plymouth, Massachusetts, during her presentation at the 2024 virtual EquiSummit conference, held Dec. 3-4. These microorganisms include bacteria that break down fiber and protein and produce B vitamins and vitamin K; fungi that break down fiber; protozoa and archaea that produce methane gas; and viruses and parasites, she added.

“The microbiome is unique to each horse, like a fingerprint,” said Schatz. “And many variables affect that horse’s microbiome, including diet as well as age, weather, management, illness such as metabolic syndrome, stress, and medications. Unfortunately, there are pretty big gaps in our knowledge about the (equine) microbiome. But in human medicine, many diseases are associated with changes in the microbiome.”

The Microbiome and Whole-Horse Health

One of the key GI microbiome concepts to understand is the microbiome-gut-brain axis. “This means the nervous, immune, and endocrine systems are all interconnected with the intestinal microbiome, and that changes to the gastrointestinal tract can have far-reaching effects,” said Schatz.

For example, alterations in the microbiome can result in behavior changes, such as anxiety, spookiness, and even stereotypies (repetitive movements with no apparent purpose). Mental stress can also affect the microbiota, causing a vicious circle.

Implications for Equine Immunity

“Seventy percent of the body’s immune system is in the GI tract, specifically the lymphatic system, which is the immune system’s superhighway,” said Schatz. “Anything that alters the GI tract can affect the immune system.”

For example, horses with equine metabolic syndrome (EMS) have gut microbiomes that differ in microorganism diversity compared to healthy horses.

“This may contribute to the decreased immune response we see in horses with EMS, but much more research is needed,” said Schatz. Researchers working to identify different microbe types commonly found in the GI tract and better ways to obtain samples from horses have made several advancements in this field.

“We need to better understand the microbiome-gut-brain axis and better understand the implications for immunity in reference to specific diseases,” said Schatz. “Then we can focus on potentially improving treating certain conditions by supporting the microbiome.”

Maintaining Your Horse’s Microbiome

The veterinary community needs more targeted research to understand this large and powerful organ, which includes the three “otics”:

• Prebiotics: ingredients or supplements that feed the beneficial bacteria in the GI tract;
• Probiotics: live microorganisms that confer a health benefit to the host when administered at adequate concentrations; and
• Postbiotics: deliver metabolic products of the microbiota, or the attempt to deliver preconverted metabolic compounds straight to the gut.

“More research is needed on these also, so the best way to support the microbiota right now is minimizing stress, maximizing turnout on pasture, offering good-quality grass/forage, and making dietary changes slowly for both grain and hay,” said Schatz.

Additionally, monitor horses for behavior changes, changes in appetite, and signs of gastric ulcers, colic, colitis/diarrhea, laminitis, and EMS. Schatz suggested working closely with your veterinarian to address these underlying health issues early in the course of disease and having a complete physical exam performed on your horse twice annually.

Additional 2024 EquiSUMMIT Coverage:

• Rules for Feeding Horses Forage
• Tackling Equine Gastric Ulcers
• Investigating Prebiotics’ Role in Managing Equine Leaky Gut
• Benefits of Extruded Horse Feeds
• What Happens With the Horse’s Gut in the Heat?