DIGESTIVE PHYSIOLOGY OF THE HORSE
The digestive physiology of the horse differs from many other species in such a way that its gastrointestinal tract can be split into two sections.
- The foregut (pre-cecal) being remarkably similar to the monogastric species’ simple stomach, like a pig or a dog.
- The hind gut being comparable to that of a cow’s rumen relying on the symbiotic relationship with microorganisms to digest the fibre portion of the ration.
The importance of the equine stomach is often overlooked due to its small size and low fermentation capacity in comparison to the hindgut. However, the stomach’s upper and lower regions have an important role in equine digestion.
The upper region of the stomach also known as the ‘non-glandular region’ contains a small quantity of microorganisms, initiating the fibre fermentation process when the pH is at the desired level of 5.0 – 7.0 (towards neutral).
The lower region known as the ‘glandular region’ secretes both hydrochloric acid for feed material breakdown and pepsin enzymes to start protein digestion. As the glandular region secretes acid in varying quantities throughout the day, the pH remains acidic at 2.0 – 3.0. It also has a protective mucosal layer to prevent the acidic environment from causing damage to the stomach lining.
The pH of the two stomach regions can be maintained when the horse is able to display its natural feeding behaviour. This is due to the almost continual forage consumption both stimulating saliva production which has low levels of acid buffering activity and aiding the maintenance of a stable feed passage rate.
Digestion in the hindgut is largely microbial rather than enzymatic. This means that digestion in the hindgut is performed by billions of symbiotic bacteria which efficiently breakdown plant fibres into simpler compounds called ‘volatile fatty acids’ (VFAs) and undigested starches into lactic acid, which can then be absorbed through the gut wall as a source of energy for the horse.
The pH of 6.5-7 in the hindgut is at the optimal level for the microorganisms to work effectively. In addition, to allow the microorganisms time to act on the fibre the passage rate of feedstuffs is much slower in the hindgut when compared to the foregut (5hrs verses 35hrs on average).
The provision of starch via concentrate feed is common, yet the horse’s gastrointestinal tract regularly can’t cope with the quantity provided. Starch is digested via the enzymatic action in the foregut, however, fast passage rate and limited digestive action means that the digestive capacity of the foregut is easily exceeded. As a result, some undigested starch can pass into the hindgut where its subsequent fermentation increases the amount of lactic acid produced, which in turn decreases the pH and changes the type of microbes present in the hindgut.
The reduction in pH from 6.5-7 to 4.0-6.0 (more acidic) in the hindgut causes some of the desirable fibre-fermenting bacteria to die and stimulates the reproduction of lactate-producing bacteria in the hindgut. This extenuates the issue as lactic acid producing bacteria proliferate in an acidic environment making it more difficult for the horse to overcome the challenge.
Hindgut acidosis is linked to a variety of equine health and behaviour issues further highlighting the need to maintain gastrointestinal health.
WHAT IS HINDGUT ACIDOSIS
Hindgut acidosis is a condition in which the hindgut of the horse becomes excessively acidic. It is usually caused by too much starch in the horse’s diet, resulting in increased production of lactic acid in the lower intestinal tract.
When lactic acid levels rise, the result is a lower pH environment in the hindgut and disturbances to the microbial population.
This can also result in inflammation in the intestinal wall and decreased resistance to pathogens and toxins found in feed.
Hindgut acidosis can have meaningful consequences for your horse’s overall well-being, including decreased nutrient absorption and feed efficiency, increased risk of hindgut ulcers, and increased risk of digestive or immune complications.
SIGNS YOUR HORSE MAY HAVE ACIDOSIS
Signs that your horse could potentially have acidosis of the hind gut, (there could also be other physical reasons, always seek a vet’s advice).
- Is your horse hot, you can’t put your leg on and you feel as though you are a passenger rather than a rider?
- Does your horse cramp behind when jumping?
- Are they funny about having a hind leg picked up?
- They aren’t muscling up quite as you would expect?
- General tightness through the body?
- Loose droppings?
- Suffer from tying up (Azoturia)?
- Poor dooer (horses though can still look really well and still have a low level of acidosis of the hind gut.) Horses that have are systemically challenged will take longer to come right.
- Are they generally grumpy or grumpy when you doing their girth or grooming in that general area?
- You have treated for ulcers with the vet and they return
SPRING GRASS & LINKS TO THE HIND GUT
During the early spring, horses are getting more turnout due to the changes in weather. However, such seasonal changes have a significant impact on the nutritional value of grass. Constant fluctuations in sugar levels make it hard for the digestive system of horses to function effectively. The following is a brief explanation as to how the changes in grass impact the hind gut and why this causes behavioural changes in many horses. The level of sugar in pasture grass varies due to several factors, including the weather, how stressed the grass is, its maturity, the time of year, and the time of day. Reactions of horses to grass growth, particularly in Spring, are due to the higher sugar level – fructans. Not only is the growth rate increasing, but also the mornings are cooler. When the sun comes up, and photosynthesis starts, the starch levels are low as this process requires heat from the sun. The fructan levels are higher but as the growth has not started, the plant stores the sugar and so the stems have particularly high levels. As the day warms up both starch and sugars are increasing but more in balance. When night comes the process stops – neither sugar nor starch is produced.
Higher fructan levels affect horses as they have difficulty digesting these sugars which often pass undigested into the hindgut where they can cause hindgut acidosis. This causes discomfort, even pain. So, their behaviour changes, although actual responses will vary as horses are all different, hypervigilant behaviour e.g spookiness, excess energy etc is common. Some may seem tense, touchy and girthy or just generally uncomfortable. It is mainly the discomfort from the unprocessed feed in the hindgut that causes the biggest reactions. By supporting the hind gut through during periods of change, within our control or otherwise, the horse can digest forage more effectively thus reducing the amount of acid build up leading to discomfort. By doing so we can keep horses more comfortable and more able to perform to their best ability.
Atlas of Topographical anatomy of the domestic animals, P. Popesko., 2008.
Luthersson N., Hou Nielson K., Harris P., et al., 2009. Equine gastric ulceration syndrome (EGUS) in 201 horses in Denmark and the influence of age, sex, temperament, breed and workload. Equine Vet J 41, 619-624.
Luthersson N. and Nadeau J.A., 2013. Gastric ulceration: Equine Applied and Clinical Nutrition, health, welfare and performance.
Rabuffo, T.S., Orsini, J.A., Sullivan E., et al., 2002. Associations between age, sex and prevalence of gastric ulceration in standardbred racehorses in training. J Am Vet Med Assoc 221, 1156-1159.