Hindgut acidosis in horses - symptoms and treatment

Gut health! It is spoken about about widely today in human health and the far reaching effects of the health of our ‘gut’ on our overall health are quite staggering. Researchers are now showing that our 100 trillion gut bacteria (that is a lot, in fact they outnumber our human cells by about 10 times, so we are kind of their human as opposed to them being our bacteria) influence our health dramatically. On the upside, a healthy population of gut bacteria can protect us from certain disease pathogens, give us useful nutrients like vitamins and volatile fatty acids, have a positive impact on our metabolism and assist our immune responses.

BUT, on the downside, an ‘unhealthy’ bacterial population can lead to issues like inflammatory bowel disease, obesity, diabetes, irritable bowel syndrome, certain allergies, mental health issues, inflammatory diseases including cardiovascular disease, asthma and the list goes on …

While there are many factors that will influence what your gut bacterial population will look like and how ‘healthy’ it is, diet is a major determinant of gut bacterial population health… for you and your horse.


Your horse’s gut

Horses evolved as grazing animals that needed to be able to survive on high fibre diets. Like us, horses don’t have the enzymes necessary to chop up and digest fibre by themselves, so they forged a ‘symbiotic’ relationship with gazillions (very technical term) of bacteria, with the unwritten deal going something like ‘we will give you somewhere warm, dark, wet and oxygen free to live with a constant source of food if you give us the left-overs of what you don’t need from the high-fibre forages we give you to ferment’. And I have to say, the system works well!

Horses dutifully guzzle down large amounts of high-fibre forage and the bacteria ferment them in the horse’s hindgut, providing the horses with a supply of volatile fatty acids that meet a large portion (70% or more) of a horses daily requirement for calories. The bacteria also provide horses with vitamins like biotin, B-group vitamins including thiamine and vitamin K and possibly also make small contributions to amino acid requirements.

And so it is, horses are hindgut fibre fermenters, and when the system is working well, horses will be very healthy. In fact, as a nutritionist I firmly believe that maintaining a healthy gut, and especially a healthy hindgut should be your number 1 consideration when choosing what you are going to feed your horse. Before anything goes in your horse’s mouth you should be asking yourself the question ‘how is this going to affect my horses gut’ or more specifically how is it going to affect the gut bacteria.


Bacterial imbalance disasters

While we don’t have as much information on how bacterial imbalances affect our horse’s health as we currently do for human health, the catastrophic consequences of an unhealthy hindgut bacterial population have been well understood for quite some time now.

Get the bacteria out of balance and you can end up with:

·         Laminitis

·         Colic

·         Systemic acidosis

·         Endotoxaemia

·         Behavioural changes

·         Vitamin deficiencies

·         Loss of appetite

·         Weak, slow growing hooves

·         Poor feed use efficiency (meaning you need to feed a small mountain of feed for your horse to maintain weight); and

·         Weight loss

I am sure there will be other consequences like poor immune function that we just don’t fully understand yet. And it has been shown in humans that bacterial populations are linked to insulin sensitivity so it is reasonable to suspect that bacterial populations play a role in the equine metabolic syndrome and insulin resistance that are seen all too often in horses and ponies these days.


What causes bacterial imbalances

There are two main things that can cause undesirable changes to the population of bacteria in your horse’s hindgut. These are:

1.       Antibiotics – I have seen antibiotics described as ‘dropping an atomic bomb on your gut bacteria’ in human health. And while legitimate use of antibiotics is often essential, their effect on gut bacteria is certainly something to be aware of.

2.       Diet – What you feed and how you feed have a huge influence on how healthy your horse’s gut bacterial population will be. To fully understand how, we should first look at what the bacterial population broadly looks like.


The hindgut bacterial populations

There are three major groups or families of bacteria in your horse’s hindgut. These are:

1.       The fibre fermenters (fibrolytic bacteria)

2.       The starch and sugar fermenters

3.       Lactate utilising bacteria

When a horse is eating a low sugar, low starch diet that is high in fibre (for example when it is grazing native pastures) the fibre fermenting bacteria dominate the hindgut bacterial populations. These little critters ferment fibre efficiently yet slowly and do a wonderful job of providing a horse with calories and vitamins, all the while keeping the hindgut environment at a lovely stable pH, very close to neutral.

When large amounts of starch or sugars like fructan are suddenly dumped in the hindgut however, the starch and sugar fermenting bacteria essentially have a party and go to town rapidly fermenting their newly arrived favourite foods. Their population increases rapidly in numbers and the lactic acid they produce starts to accumulate, lowering the pH in the hindgut environment. If the starch and sugar continues to be delivered to the hindgut they can drop the hindgut pH so low that the fibre fermenters, who don’t like acidic environments, will begin to die off (and cause endotoxemia).

There is however a ‘safety catch’ in play, and these are the lactate utilising bacteria, who, when lactic acid starts to be produced by the starch and sugar fermenters, also now have access to their favourite food (lactic acid) and they too begin to increase their population numbers. These helpful bacteria take the lactic acid being produced (which a horse cannot absorb or use in any way) and convert it to volatile fatty acids that can be absorbed and used by the horse for calories. In doing so, they keep the hindgut pH at a more neutral level and can prevent the consequences of an acidic hindgut.

HOWEVER, the safety catch doesn’t always work. First, a study byBiddle et al. (2013) found that the bacterial populations in different horses react very differently under the same conditions. Specifically they showed that some horses simply may not have a useful increase in lactate utilising bacteria when starch and/or lactate are provided for their bacterial populations to ferment.

Second, the proliferation of starch and sugar fermenting bacteria and their production of lactic acid can be exceedingly fast. Too fast sometimes for the lactate utilising bacteria to increase their numbers enough to prevent a catastrophic accumulation of lactic acid in the hindgut. So again, the safety catch fails.


Causes of bacterial imbalances

These undesirable shifts in bacteria when starch and sugars are delivered to the hindgut can occur when:

1.       Raw grains are fed! – Grains like corn, wheat and barley contain a lot of starch (between 60 and 75% of their total weight). BUT when fed uncooked, ¾ of the starch will travel undigested through the small intestine and get dumped in the hindgut where it will be gleefully fermented by the starch and sugar fermenting bacteria.

2.       Pastures accumulate fructans – many of our modern day ‘improved’ pastures have been purposefully bred to accumulate large amounts of fermentable sugars, including fructan. When horses graze these pastures it a lot of this ‘water soluble carbohydrate’ ends up in the hindgut where again it will be fermented rapidly by the starch and sugar fermenting bacteria.

3.       Starchy grain based feeds are fed in large meals  – Even if you select a feed with starch that has been cooked (more on this below) and is digestible, you still need to be really careful about how you feed it. A horse only has a very small stomach. If you feed more than the stomach can hold, your horse loses its ability to release that feed slowly into the small intestine for it to be digested and absorbed fully before it reaches the hindgut.  If you feed too much at once, you physically force feed too quickly through the small intestine and it will end up being delivered, starch and all into the hindgut where again, it will be fermented by the undesirable starch and sugar fermenting bacteria.


Keeping the hindgut bacteria ‘healthy’

Based on what causes the hindgut bacteria to get out of kilter, here are the things to do to keep them in balance:

1.       Feed lots of fibre! This is absolutely critical. Keeping your horse’s hindgut full of fibre from low sugar pasture, hay and/or chaff means that the fibre fermenting bacteria always have lots of their favourite food to eat.

2.       Limit grazing to the wee early hours of the morning. Pasture plants make their own sugars and starch (as their own source of food, it is very clever when you think about it!) during the day using a process called photosynthesis (which requires sunlight). So over the course of a day, sugar and starch levels will increase. Then the plant will burn some of the sugar and starch up overnight to stay alive. This means, sugar and starch levels are lowest in the very early hours of the morning.  If your pastures have a tendency to accumulate starch and sugars and these are causing issues for your horse, grazing very early in the morning (before sunrise until one to two hours after sunrise) will give access to the lowest sugar and starch pasture and reduce the negative impact pasture has on hindgut bacterial populations.

3.       Feed additional calories as fibre based, low starch feeds where possible. These feeds use ‘high-energy’ or super-fibres like lupins hulls and beet pulp. These fibres are fermented in the hindgut by the fibre fermenting bacteria and completely avoid any risk that starch may be delivered to the hindgut. Oils are also a useful source of additional calories should you need them.

4.       If you do feed grains, feed well-cooked grains! Not all horses do well on purely fibre based diets and grains in these situations are very useful. BUT, they absolutely must be well-cooked. My PhD focussed on the best ways to cook grain to make them easy for a horse to digest in the small intestine and therefor avoid starch being delivered to the hindgut (Richards 2003). What we found was that grains cooked via extrusion were always more digestible than grains cooked via other common methods of processing, including pelleting, dry micronization and steam flaking or steam rolling. And extruded grains are also more consistently digestible between batches of feed than other methods of cooking.

5.       Feed grain based feeds in small meals. Feeding small meals means your horse will have the best chance possible of digesting all of the starch contained in the grain based feed in their small intestine so only very minimal amounts end up in the hindgut.

6.       Introduce grains based feeds slowly. It seems to take a couple of weeks for the starch digesting enzymes in a horse’s small intestine to fully ramp up. By introducing grain based feeds slowly you give the small intestine time to adapt to digesting starch and therefore avoid starch being delivered to the starch and sugar fermenting bacteria in the hindgut.


Healthy Bacteria, Healthy Horse

If you can feed in a way that will keep your horse’s bacteria healthy and in balance your horse will be healthier in so many ways. Plus, their behaviour will be calmer and they will use the feed you give them more efficiently which ultimately makes them less expensive to keep. With so many benefits it truly makes sense to pay a lot of attention to how what you are feeding (and indeed what you eat!) is affecting gut bacteria.


Dr Nerida Richards
Equilize Horse Nutrition Pty Ltd


Biddle A.S., Black S.J. & Blanchard J.L. (2013) An <italic>In Vitro</italic> Model of the Horse Gut Microbiome Enables Identification of Lactate-Utilizing Bacteria That Differentially Respond to Starch Induction. PLoS ONE 8, e77599.

Richards N. (2003) Enhancing starch digestion in the equine small intestine. In: School of Rural Science. University of New England, Armidale, NSWAustralia.