Digestive Structure
The horse’s digestive structure is rather unique when compared with other species that have evolved to eat forages such as grass, this due to the fact that the horse is a hindgut fermenter.  Meaning the area of their digestive system that is responsible for housing the bacteria required to breakdown fibre is in the rear portion of the digestive track, rather than in an adapted stomach.   

The Horses Stomach
The stomach sits between the oesophagus and the small intestines.  In a 500kg horse it is only about the size of a rugby ball, and unlike our stomachs it has not evolved to go through phases of being completely empty or full.  Instead, the horse’s stomach is designed to cope with an almost constant supply of forages with relatively low nutritional value.  Food passes from the mouth, down the oesophagus and into the stomach via the cardiac sphincter (opening). 

The stomach can be dived into two main sections, the squamous and glandular regions.  The squamous region is the top portion of the stomach, and this area can be vulnerable to damage from stomach acid.  As it does not have a protective mucous lining, which the glandular region does.   

In the horse’s stomach acid is continuously produced and it should sit in the lower region, known as the Glandular region.  This region is  protected from the corrosive effect of the acid by a thick layer of a mucous lining.  The line between these two regions is called the Margo Plicatus and is a common site for ulceration.   Another area of the stomach that can experience ulceration is the region near the exit of the stomach, known as the Pyloric region.  Food passes through this region, out of the stomach and into the small intestines. 

EGUS
Equine Gastric Ulcer Syndrome (EGUS) occurs when there’s damage in the form of ulceration to the stomach wall.  There are many causes and mechanisms behind the types of ulceration, but they can be dependent on which region of the stomach the ulcer forms.  Ulceration in the squamous region can be due to excessive exposure to the stomach acid.  Common causes of this can be insufficient fibre, or if it’s provided in a manner that leaves periods of four plus hours without anything to chew on.  Higher work levels have been shown increase the prevalence of ulcers within the squamous region, due in part to the splashing of acid up in to this sensitive region.  Ulceration in the glandular region is more likely to be due to a failure in the production of the protective mucous lining.  This can be due to some diseases and even some medications, as well as a strong link with increased stress levels.  The causes for ulceration within the Pyloric region are less understood, but feeding ‘hard, spikey’ chaff s have been linked with Pyloric ulcer formation.  Diets of excessive sugar and starches have also been linked with an increased likelihood of a horse developing EGUS.              

I often hear owners confusing Pyloric ulcers with hindgut ulcers, and I wonder if this is due to the fact that the Pyloric region is at the back of the stomach.  But, the stomach and the hindgut are actually separated by approximately 70ft of small intestines, which sits between the stomach and the hindgut.

Hindgut
The hindgut in the horse consists of the ceacum, large colon and small colon, and it houses the microorganisms that are responsible for breaking down fibre and converting it into an energy source for the horse.  In a healthy state the hindgut has a neutral or even slightly alkaline Ph.  However, disturbances in the bacterial populations within it can cause an increase in the levels of a group of bacteria that produce lactic acid.  This then causes the environment within the hindgut to become acidic and if significant this can result in a condition called hindgut acidosis.

Hindgut Acidosis
Unlike the  diagnosis of EGUS, hindgut acidosis can be problematic to confirm as it’s not possible to reach this region with a veterinary scope. Diagnosis is more frequently achieved with the identification of symptoms, management of the bacterial populations within the hindgut and elimination of the symptoms.  These symptoms  can vary but include weight-loss, poor apatite, poor coat or hoof quality, poor performance, slow recovery, diarrhoea, girthy or spooky behaviours, slow healing rates and even increased infection rates.  Causes again can be poor fibre provisions, excessive starch or sugar in the diet, but also the feeding of unprocessed grains with low digestibility, rapid changes to a diet including fibre sources, certain infections and even some medications. 

Feeding and supplementing for digestive health 
Correct feeding supports the health and function of the digestive system.  Ensuring sufficient fibre (a minimum of 2% body weight per day) is provided aids with the correct movement of the digestive tract.  The fibre also needs to be fed in a manner to ensure that it lasts long enough.  In the case of good doers or those who guzzle their feeds the use of small holed nets such as a Trickle net to supply the hay or haylage can prove a useful tool.  Preventing periods of  four plus hours with no available fibre from occurring, and unhealthy concentrations of acid from building up in the stomach.  

Excessive starch and sugar has also been linked with both EGUS and Hindgut Acidosis, this can be avoided by controlling the levels fed at any one time.  The deemed safe upper intake of sugar and starch for a horses with digestive issues is 1g sugar and starch/Kg body weight/meal, so a maximum of 500g sugar and starch for a 500kg horse in any single bucket feed.   

Another important feeding practise to ensure a healthy digestive system, is that any changes to the diet, including forages, should be done so slowly.  Taking 1-2 weeks to change or introduce a new feed source allows for the required time for the bacteria in the hindgut to adapt. 

This transition period will help to prevent rapid disruption to the bacterial populations and it will also continue to support a neutral or alkaline environment within the caecum and colon.     

In addition there is significant evidence for the use of Yeast as a supplement to support a horses digestive system.  Yeast is frequently included in compound feeds or fed as a separate supplement.  Yeast works as a probiotic, supporting the bacteria within the hindgut and promoting the growth of the healthy bacteria.  Although the effectiveness of the use of yeast will depend on the cause of the digestive issue.  If the issue is EGUS the use of yeast will not directly support the stomach, even in the case of pyloric ulcers, as the bacteria which it supports are housed in the hindgut and not the stomach.  However, if the issues is due to the bacteria in the hindgut producing lactic acid then the supplementation of yeast could help to stabilise a healthy bacteria population.  Reducing the acid levels within the caecum and colon and alleviating the associated symptoms.  Improving digestion, vitamin B production, apatite, body condition, immune responses and even performance.

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