Hindgut acidosis in horses

You’ve most likely heard of the ‘low-starch’ trend in horse feeds, but what are the health reasons and benefits behind this trend? Many health disorders, such as laminitis and colic, are associated with hindgut acidosis which results from a starch overload in the digestive system. Hindgut acidosis is an excessive acidity in the horse’s hindgut, a drastic drop of the pH value, most commonly caused by a high grain and low forage diet. More than 60% of performance horses suffer from hindgut acidosis. This article aims to provide you with a better understanding of what hindgut acidosis is and why preventing it should be a focus point in all feeding regimes.

The hindgut ecosystem Horses evolved as herbivores, eating high-fibre diets to which their gastrointestinal tract adapted to. The microbial population in the horse’s hindgut ferments fibre, converting it into utilisable energy through the production of volatile fatty acids (VFAs). Through domestication we have modified their diet, feeding energy dense feeds such as cereal grains that are rich in starch. Starch is digested by enzymes in the small intestine yet the capacity to digest and absorb starch is easily exceeded (recommended 1-1.5 g/kg body weight per meal). When high levels of grain are fed at once, undigested starch moves from the small intestine into the large intestine. The fermentation of starch increases the production of lactic acid which in turn decreases the pH and changes the type of microbes that are present in the hindgut.

What is hindgut acidosis? The pH is a measurement of how acidic or basic (alkaline) a substance is on a scale from 0 to 14. It is neutral at 7; anything above is considered basic (alkaline) and anything lower is considered acidic. The pH in the hindgut is generally 6.5-7, at which good microorganisms such as fibre-fermenting bacteria, prosper. Such bacteria are critical to the digestion (release of energy yielding VFAs) and health of the horse and are intolerant of acidic environments. As the pH drops and the hindgut becomes more acidic, the fibre-fermenting bacteria are less efficient and begin to die-off (pH<6), releasing endotoxins. Endotoxins are toxins that are part of the cell wall of bacteria and are released when bacteria die as the cell wall breaks apart. On the other hand, lactic acid-producing and lactic acid–utilizing bacteria proliferate in a more acidic environment. It is a vicious circle as the more acid these bacteria produce, the more acidic the hindgut becomes (the lower the pH) and the more they thrive. In severe cases 50-90% of the total acids (VFAs and lactic acid) in the hindgut may be lactic acid. The increased acidity in the hindgut damages the mucosa (gastric wall) which absorbs the endotoxins released, letting them enter the general circulation. Endotoxins initiate a series of inflammatory responses and can trigger laminitis, amongst other disorders.

Clinical Signs and Diagnosis Horses with this abnormally high acidity (acidosis) in their hindgut (caecum and colon) may exhibit several signs including:

• Decreased appetite
• Performance/condition not relative to the amount of feed being consumed
• Colic symptoms
• Stereotypies (e.g. crib-biting, wind-sucking, weaving)
• Hindgut ulcers
• Slower recovery from exercise and tying-up
• Increased susceptibility to laminitis and colic
• Reduced performance

A diagnosis of hindgut acidosis is often over-looked as the above signs are not considered as severe as symptoms of associated disorders such as ulcers, colic and laminitis. Testing the pH of your horse’s manure with pH strips is an easy way to diagnose hindgut acidosis. This can in turn enable treatment before severe disorders develop.

Treatment and Prevention To treat hindgut acidosis, the root causes for this condition must be addressed. The following recommendations aid as preventive measures:

• Starch intake should be limited (1-1.5 g/kg body weight of your horse per meal) (e.g.675g for a 450kg horse)
• Grain meals should be small and fed multiple times a day, not exceeding more than 2.5kg
• Grains should be processed, preferably micronized or extruded. Thermally-processed (pelleted, extruded, micronized) feeds have a starchdigestibility up to 75%higher than the digestibility of whole grains. The greater the digestibility of the grain the more nutrients are available to the horse.
• Ad lib access to forage such as pasture grass or hay at a minimum intake of 1.5% of the horse’s body weight in forage daily.

Gastric ulcer treatments such as Omeprazole do not provide relief for hindgut ulcers or acidosis as they’ve lost their efficacy by the time they reach the hindgut. Similarly, sodium bicarbonate can attenuate the effects of hindgut acidosis as it buffers the acid produced, but only if it has a protective coating, allowing it to reach the hindgut. Live cell yeast used in HYGAIN feeds can help support the normal microbial population of the hindgut and thus avoid hindgut acidosis. The key point to remember is that prevention is the best treatment for hindgut acidosis.