Food for Thought

Over millions of years of evolution, all aspects of the digestive systems of herbivores have undergone extensive modifications to be able to convert plant cellulose to energy. The horse is no exception.

Specialized teeth & muzzle

Horses have evolved with specialized teeth. The premolars and molars are enlarged and have ridged surfaces that act as a grindstone, breaking down the plant cell walls and making the nutrients contained within them available to the horse. The higher crowned, wider, and less angled incisors meet squarely and allow for improved cropping action. The amount of chewing required over the lifetime of a horse combined with the abrasiveness of soil and sand ingested while grazing means the teeth are prone to rapid wear, so a horse’s teeth continue to grow throughout it’s life.

Along with the changes in the shape and length of the chewing teeth and incisors, changes to the length and angle of the muzzle are evident when the skulls of prehistoric horses are compared alongside the modern horse (Equus Caballus). While there were subtle variations in muzzle angle and length in earlier stages of evolution, the characteristic long, angled muzzle of Equus Caballus first became evident in the Merychippus, which evolved from previous horse-like animals about 17 million years ago. It was also the first known grazing horse.

This elongated muzzle is advantageous to the horse because it enables it to maintain a view of the horizon and surrounding area while it’s head is lowered in a grazing position, enabling it to be constantly on the look-out for predators.  

The fermentation chamber

No mammal is naturally able to digest plant cellulose due to the inability to produce the enzyme cellulase. To compensate for this, the guts of herbivorous animals are unique in that they have adapted special fermentation chambers. These fermentation chambers, the cecum in the case of horses, rhinoceros, and tapirs, contain cellulase producing bacteria that break down the cellulose in the plant walls and convert it to energy. However, the digestion of plant matter provides far less usable energy than animal protein so more of it must be consumed. 

The importance of grazing

A horse would naturally graze for 16 to 20 hours a day, and can survive on grass and hay alone. However, modern management techniques largely prevent this from happening because many horses are housed in barns or paddocks with limited access to natural forage. Rather, it has become the norm to expect horses to adapt to a human-like eating schedule, with a portion of grain in the morning and evening, and hay in between. This diet is sometimes compensated with various supplements in an effort to maintain a healthy, balanced diet for the horse.

Having evolved over millions of years, the horse’s digestive system is not well-suited to the habits of well-meaning humans. Given the small size of the its stomach, it is essential that the horse eat small amounts of food frequently, as it is not equipped to handle large amounts of food at once, and doing so could have severe health consequences, including ulcers, colic, and laminitis.

Tips for digestive health

Ideally, the horse needs to graze almost constantly and failing that, it should be fed several small meals throughout the day on a set schedule. Basic guidelines for feeding and maintaining the digestive health of a horse are as follows:

  1. Ensure clean, fresh water is always available.
  2. Feed little amounts of food often and adjust according to the work load, temperament, and condition of the horse.
  3. Establish a routine and try to maintain the same feeding hours each day, allowing for seasonal variations to account for changes in daylight hours and time spent at pasture.
  4. Feed adequate roughage consisting of grass and good quality hay.
  5. A horse should have either grass or at least some succulent food such as carrots, turnips, beets or apples every day.

Resources

Bennett, Deb and Robert S. Hoffman. “Mammalian Species: Equus caballus.” American Society of Mammalogists. Published 3 December 1999. Retrieved from http://www.science.smith.edu/msi/pdf/i0076-3519-628-01-0001.pdf 

Florida Museum of Natural History: Merychippus. Retrieved from https://www.flmnh.ufl.edu/fhc/merychippus.htm

Janis, C. (1976). “The Evolutionary Strategy of the Equidae and the Origins of Rumen and Cecal Digestion”. Evolution 30(4) December 1976. p. 757-774

Landels, J. 2012. “What’s on the Menu?” Academie Duello Accessed 2 Oct. 2016 http://www.academieduello.com/news-blog/whats-on-the-menu-2/  

Landels, J. 2016. “The Grain of the Matter.” Academie Duello Accessed 2 Oct. 2016 http://www.academieduello.com/news-blog/the-grain-of-the-matter/

MacFadden, B. (1992). “What’s the Use? Functional morphology of feeding and locomotion.” Fossil Horses: Systematics, Paleobiology, and Evolution of the Family Equidae. Cambridge University Press, 1999, p.241

Texas Equine Dentistry: Equine Skull Anatomy, Interesting Facts. Retrieved from http://www.texasequinedentist.com/blog/2013/06/equine-skull-anatomy-interesting-facts/

University of Guelph. 2016. Functional Anatomy, Week 1 Presentation: The Gastrointestinal Tract – The Equine Digestive System

All About Gastric Ulcers

The horse’s digestive system has evolved over thousands of years and is not well-suited to the habits of well-meaning humans. Given the small size of the horse’s stomach, it’s essential that it eat small amounts of food frequently, as it is not equipped to handle large amounts of food at once. Ideally, the horse needs to graze almost constantly and failing that, it should be fed several small meals throughout the day on a set schedule.

There are a number of factors to consider when developing management techniques for your horse. Like humans, each horse will have different metabolic requirements which will vary throughout the year. Food requirements will vary based on work, temperament, and body condition of the horse. A routine should be established and maintained to make sure the horse is eating frequently throughout the day, allowing for seasonal variations to account for changes in daylight hours and time spent at pasture.

The highly acidic stomach of the horse

The horse’s stomach is lined by a mucous membrane called the gastric mucosa. It produces a highly acidic, enzymatic secretion that initiates the digestion of protein. Muscular activity in the stomach mixes boluses of food with the digestive fluid until they are reduced to small particles and solubilized before passing into the small intestine.

However, the gastric mucosa only protects part of the horse’s stomach. The stomach of a horse is divided into two regions. At the top is the non-glandular squamous or esophageal region that is lined by the same tissue that is found in the esophagus. The bottom region of the stomach is the glandular gastric mucosa, which is similar to the human stomach. But unlike the human stomach, which only produces acid in response to food, the horse’s stomach is constantly producing acid, up to 9 gallons per day.

Manageable causes of gastric ulcers

The primary causes of gastric ulcers are a combination of infrequent feeding, type of feed, exercise and travel.

Importance of grazing: Under natural grazing conditions, where the horse has a near constant flow of food through its system, the stomach acid is neutralized by food and saliva. But under modern management practices, a horse’s eating patterns are vastly altered and the prevalence of gastric ulcers is high. The highest rates of occurrence are in performance and show horses and any other horse that is fed a restricted and/or grain-based diet. Foals are also highly susceptible. Their stomachs begin to produce gastric acid as young as 2 days of age. 

Type of feed: Roughage is the best thing a horse can eat to help mitigate ulcers. Because of the amount of chewing it requires, eating roughage stimulates more saliva production which in turn helps neutralize stomach acid. In contrast, concentrated feeds and grain increase acid production in the stomach and require less chewing, which means less saliva is produced.

Exercise & Travel: There are many disruptions to the natural grazing routine of the horse. When engaged in exercise, particularly in the case of racehorses and show horses, they are prevented from grazing and are often forced to fast for long periods of time. The potential for damage in these situations is two-fold. In addition to the normal buildup of stomach acid, various stresses (exercise, transit) can increase acid production and decrease blood flow to the stomach, making it more susceptible to injury.

Overeating: Although less frequent, ulcers can also develop if the horse’s stomach is too full, thereby displacing gastric acid and causing it to come into contact with the upper, more delicate region of the stomach.

Symptoms

Ulcers occur most frequently in the upper region of the stomach, and occasionally in the lower region and the small intestine. Signs that your horse may have an ulcer include:

  • Reduced appetite
  • Slow eating
  • Change in attitude
  • Mental dullness
  • Irritability
  • Reluctance to work
  • Poor physical condition
  • Poor hair coat
  • Excessive time spent lying die
  • Mild colic
  • Weight loss
  • Loose feces
Diagnosis and Treatment

Gastric ulcers are diagnosed with a gastric endoscopy (gastroscopy) to confirm the presence, severity, and location of the ulceration. This is a minimally invasive procedure that allows the veterinarian to view the esophagus, both regions of the stomach and a segment of the small intestine.

Treatment consists of a combination of management practices and medication. The two main types of medication used in the treatment of ulcers are H2 blockers and acid pump inhibitors. H2 blockers partially limit acid production by blocking the histamines that stimulate acid production in the stomach. Acid pump inhibitors completely stop the production of stomach acid.

In addition to medications, it is important to address the root causes of ulcers and adapt management practices by increasing the amount of roughage in the diet, allowing the horse to graze for at least part of the day, increasing the frequency of feedings, and avoiding or decreasing the amount of grain. Limiting stressful situations like intense training and transportation will also be beneficial.

References

Clyde Vet Group. Ulcers – “How Susceptible is Your Horse?” Retrieved from http://www.clydevetgroup.co.uk/equine/newsletters/apr07.htm

Foster & Smith. “Equine Gastric Ulcer Syndrome.” Retrieved from http://www.drsfostersmith.com/pic/article.cfm?aid=1587 

Livesey, M. “The Structure, Function and Dysfunction of the Equine Digestive System.” University of Guelph Equine Research Centre. Date unknown.

Nieto, J. (2012). “Diagnosing and Treating Gastric Ulcers in Horses.” Retrieved from http://www.thehorse.com/articles/30850/diagnosing-and-treating-gastric-ulcers-in-horses

 

Curious About Colic?

There are many types of colic a horse can suffer from and in the majority of cases, a specific cause may not be identifiable. These cases are referred to as idiopathic and include mild colic, impaction, gas/spasmodic colic. The most common types of non-idiopathic colic (cause known) are enteritis, displacement, torsion, and sand colic.

Impaction – This is an obstruction in the horse’s colon resulting from an accumulation of sand, dirt, feed, or some other indigestible material, including enteroliths [1] and sand colic [2]. In most cases, dehydration or poor quality feed result in impactions.  

Spasmodic/gas – This can be caused by a buildup of excess gases or fluids in the colon, or could simply be an involuntary contraction or spasm. Improper fermentation of food, inflammation, poor parasite management, colonic or gastric ulcers, and stress can also cause spasmodic colic.

Enteritis – Quite simply, enteritis is an inflammation of the intestine caused by infection, bacteria, and virus.

Displacement and Torsion –  The suspension of the small intestine from the mesentery and the unfixed nature of most of the large intestine make it very susceptible to displacement and torsions. When displacement occurs, a portion of the intestine has moved into an unnatural position in the abdomen and a twist in the intestines is the most insidious form of colic. These types of colic result in a complete blockage in the intestine and require immediate surgery.  

Management Techniques

Establishing and maintaining a healthy diet and feeding routine for your horse is the most important and effective prevention tool for colic.  Feed several small meals throughout the day consisting primarily of roughage in the form of grass and hay, limit grain where possible, allow for foraging, and provide a constant supply of fresh water.

Other factors to consider are parasite management and exercise. A regular deworming schedule is required to minimize the parasite load in the horse’s gut. Tapeworms attach near the junction of the small intestine and cecum, damaging the intestinal mucosa and causing nerve degeneration. A large concentration of tapeworms (or other parasites) can obstruct the bowel. Even smaller populations of tapeworms can cause ileal impaction, spasmodic colic, and even intussusception and rupture.  

Exercise also has a preventative role. Horses are not meant to spend extended periods of time standing in stalls, and research has shown that even light physical activity like walking stimulates g.i. motility and increases the digestibility of fiber by up to 20%.

Treatment

If your horse is exhibiting symptoms of colic, the first thing you should do is call your vet. If the horse is lying quietly, it is best to let it rest rather than walk, potentially causing more discomfort and draining its energy. On the other hand, if the horse is rolling, pawing or thrashing about in response to the pain, then getting it to walk quietly is a good course of action because not only could the horse injure itself by getting a leg or foot caught in a fence or under a stall door, it could also go into shock from overexertion.

Banamine can be administered in instances of spasmodic (stress-induced) colic, where there is no impaction suspected. The problem with owner-administered Banamine is that owners may not be aware of the potential side-effects and correct dosing. Also, if there is an impaction, the Banamine could ease the discomfort enough to mask the symptoms, leading to further complications.

The best-case scenario any horse owner can hope for where colic is involved is that it is caught early and is a result of a) stress (spasmodic) which will most likely pass with minimal treatment – Banamine may be administered to alleviate pain and reduce inflammation, or b) a blockage, which, if minimal, can be remedied by a dose of mineral oil.

Worst-case scenario, the horse has a twisted intestine (torsion) and will require surgery. While surgery has a high success rate if the colic is caught early enough, it is quite costly and puts the owner in the difficult position of choosing between surgery and euthanasia.

Bottom line, calling the vet at the first sign of colic is of the utmost importance to increase the chances of a successful outcome.

Notes

[1] An enterolith is a stone-like formation of minerals that forms around a foreign object that a horse has ingested, similar to a gall stone in humans. While an enterolith can grow to a significant size, reaching a weight of up to 15 pounds, they will sometimes pass completely through the horse’s system and seldom completely block the passage of food through the g.i. tract. They can, however, cause mild bouts of impaction colic if they migrate to an area where food can’t pass.

[2] Horses in sandy regions are susceptible to impaction colic when large amounts of sand (upwards of 30 pounds) collects in the g.i. tract. Preventative measures include supplements and elevating hay and grain off of the ground.

References

Brown, J. (2012). “Banamine.” Horse & Pony, February 2012.

Lenz, T. (2012). “Horse Health: Tapeworms.” American Association of Equine Practitioners, April 2012.

Loving, N. (2006). “ Impaction Colic: Blocking the Way.” The Horse, May 2006.

Sellnow, L. (1999). “Colic: To Walk or Not to Walk.” The Horse, September 1999.

The Equine Research Centre. (2016). “The Colic Fact Sheet.” Accessed 20 Sept. 2016

Unknown. (2016). “Understanding Enteroliths.” Equus Magazine. Accessed 21 Sept. 2016.