Getting back to basics might be the key to keeping the largest portion of the horse’s GI tract functioning properly
Horses are powerful, athletic animals. Their digestive systems, however, are delicate compared to those of most other types of livestock. Ruminants such as cattle and sheep have multicompartment stomachs. Saliva created by chewing a cud processes food in the front half of ruminants’ digestive tracts. Horses, however, rely on a metabolically complex fermentation process. And because horses only have one stomach, most of that fermentation occurs in the back part or hindgut.
Despite making up the largest portion of a horse’s gastrointestinal (GI) tract, the hindgut, which includes the cecum and large colon (or large intestine), often receives far less attention from owners than the stomach or small intestine, says Kenneth Kopp, DVM, a consulting veterinarian based in St. Louis, Missouri.
“The hindgut is about 25 gallons in a 1,000-pound horse—that’s huge compared to the (2-4-gallon) stomach,” he says. “The stomach is only 10% of the GI tract, but there’s such a focus there.” Billions of microorganisms, including protozoa, fungi, and bacteria, live in the hindgut. Their job is to convert carbohydrates into fatty acids and provide the horse with energy. Feed or forage might spend as much as 48 hours in the hindgut compared to a few hours or less in the small intestine.
“Ideally, by the time material gets to the hindgut, water-soluble carbohydrates and easily digestible proteins are already broken down and absorbed,” says Amy Biddle, PhD, assistant professor of animal science in the University of Delaware’s Department of Animal and Food Sciences, in Newark. “The fibrous part of feed, structural carbohydrates, are passed right through the small intestine to the hindgut, where the bacteria are really good at breaking them down and converting them to energy sources that the horse can utilize.”
The hindgut also absorbs short-chain fatty acids, which Biddle estimates provide at least 46% of the horse’s energy stores. For that and other reasons it is critical to keep those microorganisms happy and functioning properly.
Although researchers, veterinarians, and nutritionists know how critical hindgut function is to overall health, they know much less about this part of the digestive system than others. Generally, they’re unable to examine the hindgut with imaging equipment, and clinicians don’t perform autopsies in horses as frequently as they do in food animals. Veterinarians can use ultrasound to see small portions of the hindgut through the abdominal wall, and they can reach the colon with a GI “smart pill” endoscopic camera, but visibility is still limited. For the most part, researchers look to studies in humans and other livestock species and existing equine digestive system research to help horse owners promote a healthy hindgut.
Maintaining the Status Quo
The hindgut is naturally more basic than other parts of the digestive system, says Biddle. Large grain meals and sudden diet changes can promote the growth of lactic acid producers that rapidly shift the pH level in the hindgut and can lead to lactic acidosis, a dangerous situation in which the acidity increase kills off the beneficial bacterial population.
“This is why it is important to change feeds so slowly, so the microbes in the hindgut can keep up,” says Biddle, and so abrupt changes do not stress the system. “Especially if you’re going to change to a diet higher in nonstructural carbohydrates (simple sugars and fructans, which are readily digested), it has to be introduced slowly.”
Getting back to basics and simplifying meals is the best approach for maintaining a healthy hindgut. Ideally, you want to feed your horse a diet that’s high in forage and low in grains, says Anthony Blikslager, DVM, PhD, Dipl. ACVS, head of the department of clinical sciences and equine surgery and gastroenterology professor at North Carolina State University’s College of Veterinary Medicine, in Raleigh.
Horses evolved on ever-changing marginal grasslands. They were browsers that adapted to be grazers that rely on continual forage intake.
Blikslager says fresh grass is the ultimate forage for horses, but investing in high-quality hay is the best place to start improving the equine hindgut. He believes you can correct approximately 80% of a horse’s condition simply by adjusting the forage in the diet.
“When we see horses not holding their weight or running out of steam, that’s when we look at their diet and see if any minerals or vitamins are missing, particularly in certain (regions),” he says.
The National Research Council guidelines for nutrient requirements in horses are a good resource for building a diet around forage, Biddle says. She also recommends software programs such as FeedXL’s nutrition calculator to help design or evaluate individual diets. Performance horses, working horses, and lactating mares, for instance, have higher energy needs and, therefore, require more energy-dense diets.
“It’s all about keeping the bacteria happy, and that starts with high-quality forage as the foundation,” she says.
When Things Go Wrong
Abdominal pain, known as colic, is the most obvious sign the hindgut isn’t working well. Even mild grumpiness, especially when you touch a horse’s belly, could be symptomatic of an upset hindgut. Often, a horse that is “not cooperating” on the ground or under saddle could be feeling not quite right.
In his practice, Kopp has observed affected horses that love to eat hay but don’t like grain or consume it slowly. He has also seen horses constantly shifting weight from one hind leg to the other.
“It’s not colic, but is there low-grade inflammation in that hindgut?” he says. “When we start seeing some of those things, maybe the horse needs forage supplements for better hindgut health.”
The next most common complication is called leaky gut. The entire intestinal tract is lined with a thin layer of cells that create a barrier against stomach acid. Kopp explains that the cells line up side by side and attach in a similar construction to Velcro. If their integrity is compromised, leaking can occur.
“When we get a leak into the bloodstream from the gut, offending pathogens (disease-causing organisms) can lead to inflammation anywhere in the body,” Kopp says. “What researchers are finding is that this leaking and inflammation cycle may make horses more prone to pneumonia and mastitis (mammary gland inflammation) and even opens the possibility of allergies and autoimmune disease.”
Laminitis (a hoof disease that causes the laminae—tissues suspending the coffin bone within the hoof—to become damaged and inflamed) can be another complication of poor hindgut health. The laminar structures in the horse’s hoof are so delicate that the blood vessels cannot tolerate sudden change. If a large amount of undigested starch (e.g., from grain overload) enters the hindgut, for instance, acidosis can cause the bacteria to die and release endotoxins. If endotoxins suddenly leak into the horse’s system, they can trigger laminitis, Blikslager says.
Kopp says the most common form of hindgut-related laminitis is pasture-associated laminitis in horses with equine metabolic syndrome or pituitary pars intermedia dysfunction (aka equine Cushing’s disease), with the primary mechanism being insulin toxicity of the laminae.
“Mild leaky gut has been linked to insulin resistance and metabolic syndrome in other species and could very well be part of the problem in equines,” Kopp says. “Chronic low-grade leaky gut could help explain insulin resistance and laminitis risk in horses.”
Because we don’t fully understand leaky gut’s whole-body impact on horses, Kopp looks to human and food animal research for answers. He cites work Adam Moeser, PhD, DVM, a professor at Michigan State University College of Veterinary Medicine, has done in piglets that closely parallels studies of children and the lasting impact of early weaning.
“Findings are showing that weaning stress early in life leads to lifelong GI issues,” Kopp says. “The kids that were followed were found to have irritable bowel syndrome and chronic gut issues later in life, and Adam is seeing that in the piglets. Once they start leaking early in life, they’re more reactive to stressful situations later in life.”
What About Pre- or Probiotics?
If hindgut health relies on a balanced natural microbiome—an ecosystem of microscopic organisms—won’t feeding pre- and probiotics keep it healthy and avoid colic, leaky gut, and laminitis? Theoretically, giving the horse a prebiotic feeds the hindgut microbiome by providing a food source for the bacteria strains that already live there.
“Prebiotics typically contain yeast and fiber that are easily fermentable by bacteria and promote growth,” Biddle says.
Probiotics contain favorable bacteria and yeasts that might effect a change or encourage colonization. The biggest problem is not knowing how many of the live organisms will make it into the colon and what their physiological impact will be.
“It’s generally thought that probiotics give the bacteria a jolt in a favorable direction but then come back to a normal state,” Blikslager says. “It takes a lot to make a difference and why we tend to feed (these products) daily. Plus, no one has fully answered how many (bacteria or yeast) make it to the colon and (cause a) permanent change.”
Supplemental butyric acid, a short-chain fatty acid, is a new equine nutrition tool that could be a hindgut health game changer. It’s been shown to increase colonocyte growth, which is thought to contribute to a healthier gut barrier.
Management Practices to Support the Hindgut
While we need more research to better understand the equine hindgut, our sources agree that some management practices can influence hindgut health. For instance, it’s important to introduce or change hay just as gradually as you would transition to a new grain type.
Spreading meals throughout the day is ideal. Generally, horses receive meals twice a day because it is convenient for their caretakers. However, consuming multiple meals, especially for senior horses, is healthier.
Kopp emphasizes the importance of providing consistent fermentable fiber that feeds the hindgut bacteria. He is an advocate of chopped forage and hay cubes or pellets and has seen horses respond well to fermentable fibers such as shredded molasses-free beet pulp and soybean hulls.
Biddle encourages owners to monitor their horses’ manure to get a sense of what is normal for each animal. Horses should have well-formed fecal balls that aren’t hard. If the manure is hard, the horse might not be consuming enough water. A gritty consistency indicates the presence of sand.
“If you can get more water in the horse, it can aid in gut motility and prevent an impaction,” she says. “Soaked feeds like beet pulp, alfalfa cubes, adding some water to their grain, and giving their hay a little douse could be helpful in increasing water intake.”
Movement can also help hindgut health, says Blikslager. While not all horse temperaments or facilities are conducive to round-the-clock pasture time, it’s important to give the horse some opportunity to move every day.
Each horse is an individual, and as caretakers it’s our responsibility to feed them as such. This can admittedly be challenging on large farms feeding multiple horses in large paddocks. However, backyard horse owners and managers of smaller barns or farms with stabling have an opportunity to customize each diet to optimize hindgut health.