The equine gastrointestinal tract is a highly functioning structure that, when healthy, is extremely effective at digestion, energy conversion and nutrient absorption. However, certain aspects of the design of the equine gut can increase the risk of digestive conditions that can cause problematic symptoms. The most common digestive problems seen in equines are gastric ulcers which mainly occur in the stomach and hindgut acidosis which can affect the large intestine, caecum and colon.
Both gastric ulcers and hindgut acidosis are common in all horses, however, they are particularly high in performance horses. While there are many contributing factors to the conditions and horses can be at risk of them when feeding practices are correct, both conditions generally occur due to stress and incorrect feeding practices. Diagnosis can be tricky as both conditions can display similar symptoms, and to make diagnosis even more difficult, the conditions can also be asymptomatic, meaning no outward signs are displayed. However common signs include inappetence, grumpy behaviour, loose manure, mild colic and underperformance.
The prevalence of gastric ulcers varies in different countries, however, most studies indicate a 90% incidence of some degree of ulceration in racehorses and a 60% incidence in sport horses. Ulcers are categorised differently depending on where they occur in the stomach, and the different types often require different procedures to treat and manage them. The most common type of ulcer is squamous ulcers, which occur through prolonged exposure of the upper squamous mucosa to gastric acid. Unlike the lower glandular portion of the stomach, the squamous mucosa does not have a mucous layer and does not secrete bicarbonate onto its surface. Despite the lower region of the stomach having an additional layer of protection, ulcers can also occur here. These are known as glandular ulcers and occur due to a breakdown of the mucous layer due to a consistent acidic environment and exposure of the glandular layer to acid. The third type of ulcer is known as pyloric ulcers and occurs in the lower bend where the stomach curves to join the duodenum.
All three types of ulcers are caused by a decrease in gastric pH causing an increased acidic environment over time. Squamous ulcers are more common simply because they occur faster due to this area having less protection from acid than the other parts of the stomach. The only protection that this portion of the stomach has from gastric acid and pepsin comes from saliva production. If adequate saliva is not produced to buffer the gastric acid and coat the squamous epithelium, then gastric irritation occurs and lesions may develop.
One of the biggest contributing factors to all three types of ulcers is inadequate consumption of fibre, particularly long-stemmed roughage. Horses produce gastric acid at a constant rate and stomach pH can decrease quickly if the horse does not eat frequently. Meal feeding and prolonged periods without access to forage is the number one cause of ulcers, as consumption of forage stimulates saliva production which contains bicarbonate to buffer gastric acid and protect the non-glandular section of the stomach. Forage also works to neutralise the acid in the lower part of the stomach and prevent glandular and pyloric ulcers from forming. The pH of the gastric fluid in horses withheld from feed for several hours has consistently been measured to be 2.0 or lower (highly acidic). Horses that received free-choice grass hay for 24 hours had average gastric pH readings that were significantly higher than fasted horses. High pH readings (less acidic) in hay-fed horses should be expected since forage consumption stimulates saliva production which naturally buffers acid.
At least 1.5% of the horse’s body weight in forage-fed at a consistent rate is required for ulcer prevention. It is also preferable if the largest contribution of forage in the diet is in the long-stemmed form, such as hays and pasture, as this encourages longer chewing time and therefore increased saliva production. The type of forage fed to horses has a significant impact on acid neutralization and the incidence of gastric ulcers. While grass hay types are beneficial, lucerne provides greater buffering capacity compared to grass forages for several reasons. First, lucerne contains higher levels of protein and calcium, both of which buffer gastric acid. Also, the lucerne cell wall contains certain indigestible compounds such as lignin that give it a greater buffering capacity than grasses. Nutritionists often recommend horses are fed a small amount of lucerne hay or haylage before a workout to put a fibre mat over the acidic stomach contents and reduce acid splash.
Diets higher in grain also contribute to the risk of ulcers as grain and concentrate feeds have less of a buffering effect on the stomach, and increase gastrin production, the hormone that stimulates gastric acid production. While grain is a beneficial energy source for horses in work, it is recommended that horses at risk of ulcers receive a low grain diet. To help reduce the starch intake yet still supply enough energy for work it is recommended to provide calories through fat supplements such as oil and stabilised rice bran conditioning supplements such as KER Equi-Jewel.
Many horses with ulcers have no clinical signs. However, typical signs include poor performance, poor appetite, mild colic, behavioural changes, loose manure, and weight loss. Ulcers can lead to physiological changes that can reduce performance. For horses suspected of having ulcers, it is recommended to have a veterinarian diagnose the problem with an endoscope and treat using a proven ulcer medication. Once the ulcers are treated, aim to prevent their recurrence by correct feeding management. This includes reducing the size of grain meals (feed smaller meals more often), adding fat supplements, feeding adequate fibre, allowing some grazing time, and not working horses on an empty stomach.
While not as well-known as gastric ulcers, it is thought that hindgut acidosis is equally as common. The hindgut includes the cecum, large colon, small colon, and rectum. Because of the limited size of the horse’s foregut, food undergoing digestion spends little time there when compared to the hours it spends progressing through the hindgut. The most significant feature of the hindgut is the fragile population of microorganisms that inhabit it. Anaerobic bacteria, fungi, and protozoa coexist contentedly in the hindgut when the system is working as it should. Together the microbes’ primary responsibility is to digest fibre.
Hindgut acidosis occurs when the pH of the hindgut decreases and the delicate array of microbes are disturbed, mainly through the production of lactic acid. This mainly occurs in two ways; over-consumption of high starch concentrates or pastures rich in sugars known as fructans.
The demands placed on performance and breeding horses often mean that their energy requirements are higher than what forage alone can provide, which is where grain and concentrate feeds are utilised. Grains provide highly beneficial energy sources for horses however they are required to be fed correctly to prevent contributing to digestive conditions such as hindgut acidosis.
The delicate array of microbes that reside in the hindgut is dedicated to fibre digestion and fermentation, and microbial disturbance and a drop in pH occur when undigested starch and sugars escape digestion in the small intestine and overflow into the hindgut. The limited capacity of the stomach and small intestine means that this can happen easily and keeping grain meals small is highly important in preventing this condition from occurring. Recommendations are to feed no more than 2.5kgs of grain or concentrate feed per meal to an average 500kg horse. Correct grain processing is also essential to ensure it is digested in the foregut, and with the exception of oats, all grains require processing with heat and pressure to increase digestibility. Horses at risk of hindgut disturbances and acidosis should receive a low grain diet and be provided with alternative energy sources to meet caloric requirements such as high-quality fat and digestible fibre.
Ingestion of fructans from pasture is another significant contributing factor to hindgut acidosis. Fructans are specially adapted sugars that are found in cool-season forages and have extremely strong bonds that hold them together to form short- and long-chain carbohydrates. These bonds cannot be broken down by the normal enzymatic mechanism in the stomach and small intestine. For this reason, these easily fermented sugars pass into the hindgut, a situation that leads to the rapid production of lactic acid and a subsequent decrease in pH. The resulting accumulation of lactic acid in the hindgut is one of the most direct causes of hindgut acidosis.
If hindgut acidosis is occurring due to pasture exposure the best mode of action is to restrict grazing and offer low sugar forage alternatives such as hay to ensure adequate forage is being consumed daily.
Symptoms of hindgut acidosis can be similar to gastric ulcers and include grumpy behaviour, loose manure and poor performance. Some more concerning symptoms of the condition however include mild colic, and it is also implicated in the development of laminitis.
Various digestive aids are available for maintaining hindgut balance however one of the most effective is Kentucky Equine Research EquiShure. EquiShure is a time-released buffer that helps horses maintain a stable hindgut environment to encourage proper digestion and absorption of nutrients.
Both digestive conditions gastric ulcers and hindgut acidosis can be difficult to manage and require assistance from an experienced veterinarian and qualified equine nutritionist.
Article supplied by Luisa Wood, Equine Nutritionist