The horse has a highly specialised large intestine (caecum and colon), known commonly as the ‘hindgut’. Within the hindgut there are many billions of bacteria who live with the horse in a comfortable arrangement whereby the horse provides them with accommodation and they provide the horse with energy from fibre and a host of vitamins.
The hindgut environment is tightly controlled to allow the bacteria to thrive and do their job. High grain diets like those fed to racing horses can however quite easily upset the balance, causing mass deaths of beneficial bacteria and severely limiting hindgut functionality. The consequences of poor hindgut health can be as far reaching as weight loss to behavioural changes, poor hoof quality and loss of appetite. Needless to say, keeping the hindgut healthy is in your best interests if you want horses to perform at their best.
The hindgut and health
The hindgut plays several roles in the digestion of feed and in maintaining the overall health of a horse, including:
Fibre Digestion – horses don’t have the enzymes necessary to digest fibrous feeds like pasture and hay. However the bacteria in the hindgut are capable of digesting fibre via a process of fermentation, providing the horse with energy in the form of volatile fatty acids (VFAs). VFAs are a major source of fuel for muscles in racing horses.
Hydration and Electrolyte Balance – the hindgut and the fibrous material within it provide a reservoir of water and electrolytes for the horse which may be absorbed when needed to keep the horse hydrated.
Vitamin Supply – the bacteria that ferment fibrous feeds also produce vitamins including vitamin K and B-group vitamins like thiamine (vitamin B1) which is important for the maintenance of appetite and biotin which is critical for hoof growth.
Hindgut Acidosis – A truly messed up gut!
Hindgut acidosis describes a condition in horses where the pH in the hindgut becomes acidic. Diets that contain whole or cracked (uncooked) cereal grains like corn or barley deliver a lot of undigested starch to the hindgut. This starch is rapidly fermented by a small population of bacteria (amylolytic bacteria), resulting in an accumulation of VFAs and lactic acid and a condition known as hindgut acidosis.
A survey published in 2006 (Richards et al.) found that more than one quarter of racing thoroughbreds have hindgut acidosis. Even though it is a common disease that will severely limit racing performance, because its symptoms are often subtle, it is not well recognised in racing horses.
Acidic Hindgut = Unhealthy Horse
Hindgut acidosis has many negative implications for racing horses, including:
Reduced fibre fermentation – Fibres like hay and chaff are often thought of as ‘fillers’ for the horse. In reality, these fibres supply an important source of energy, with around 40% of a horses daily calorie needs being supplied from the fermentation of fibre. The problem is, when the hindgut becomes acidic, the bacteria responsible for fermenting fibres become inefficient and eventually start to die, meaning fibres are no longer efficiently fermented and the horse starts to miss out on this source of energy. What this ultimately means is the horse will start to lose weight.
Vitamin B1 deficiency and loss of appetite – The bacteria responsible for causing hindgut acidosis also produce a compound called thiaminase which destroys vitamin B1 (thiamine). Vitamin B1 plays a critical role in maintaining appetite, so if a horse develops hindgut acidosis it will also commonly stop eating grains, preferring instead to just eat hay and chaff. This loss of appetite commonly results in the horse losing weight and needing a spell.
Biotin deficiency and poor hoof quality – hindgut acidosis also affects the production of biotin in the hindgut and limits the growth of good, strong hoof tissue. An acidosis induced biotin deficiency is likely to be a major cause of hoof problems in racing thoroughbreds.
Colic – In severe cases of acidosis a horse may experience colic. It is also common to see soft to sloppy manure that has a pungent, acidic, vomit like smell in horses with acidosis.
Altered behaviour – horses with acidosis will commonly exhibit some form of altered behaviour which may include being generally agitated, chewing on timber, eating their bedding or manure and licking at their stalls.
Endotoxemia and metabolic acidosis – If hindgut acidosis is severe enough, the wall of the hindgut becomes damaged by the low pH conditions, allowing endotoxins and lactic acid from within the hindgut to pass through the gut wall and into the bloodstream, lymph or the peritoneal cavity.
Laminitis – It is well known that hindgut acidosis can lead to the painful inflammation of the lamella in the hoof. While laminitis is well recognised in its acute form, it is possible for horses to suffer with chronic laminitis where only mild to almost undetectable lameness occurs. Chronic laminitis may be so mild it may just cause a horse in full gallop to shorten its length of stride.
Maintaining a healthy hindgut
If a horse’s hindgut is out of kilter the horse cannot possibly work and perform to the best of its ability. It is critical that you maintain the population of beneficial bacteria in the hindgut and avoid the fermentation of starch. To do this, there are two very simple rules to follow:
NEVER feed uncooked corn or barley – The starch in corn and barley is very difficult for a horse to digest in the small intestine unless it has been cooked. Feeding corn or barley whole, cracked, crush, rolled or ground is going to mean that more than 70% of the starch contained in those grains will end up being fermented in the hindgut and causing acidosis. Corn and barley are excellent, high starch sources of energy for racing horses, but they MUST be fed cooked.
ALWAYS feed enough hay and chaff – Racing horses should be fed at least 1% of their bodyweight in hay and chaff, which is 5 kg/day for a 500 kg thoroughbred (as a guide, one biscuit of lucerne hay will weigh close to 2 kg, a 2L dipper of chaff weighs around 250 grams). Feeding less than this starves the hindgut bacteria of fibre to ferment, upsetting the hindgut environment and limiting the amount of energy and beneficial vitamins supplied to the horse.
If you are concerned about a weight disadvantage from feeding this much forage, simply reduce the amount of hay and chaff fed for 2 to 3 days leading up to a race.
Feed the hindgut
When making any feed choices for racing horses you should always first consider the impact the feed will have on the hindgut. Because the hindgut is so critically linked to a horses overall health, keeping the hindgut healthy will give a horse its best chance at racing well. Mess it up and the horse will lose its appetite, lose weight and lose its ability to race at the level it is capable. Get it really wrong and you will lose the horse entirely to colic, laminitis or endotoxemia. And getting it right really is as simple as feeding cooked grains and plenty of fibre!
Dr Nerida Richards
Equilize Horse Nutrition Pty Ltd
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