Hindgut Acidosis in Horses: Causes, Symptoms & Fixes

What Is Hindgut Acidosis in Horses?

Hindgut acidosis is a condition where the pH in a horse's large intestine — primarily the caecum and colon — drops too low, creating an excessively acidic environment. In a healthy horse, the hindgut operates at a pH of roughly 6.5 to 7.0 and is home to billions of beneficial microorganisms that ferment fibre and produce volatile fatty acids (VFAs), a crucial energy source.

When the hindgut becomes too acidic, those beneficial microbes begin to die off and are replaced by acid-producing bacteria. The consequences ripple throughout the entire body, affecting everything from behaviour to hoof health.

Hindgut acidosis is far more common than many horse owners realise. It sits on a spectrum — from mild, subclinical cases that cause vague performance or behavioural issues, to severe episodes that can trigger laminitis or colic. Understanding the causes, recognising the symptoms, and making the right dietary adjustments can make a dramatic difference to your horse's health and well-being.

What Causes Hindgut Acidosis?

The root cause of hindgut acidosis is almost always dietary: too much rapidly fermentable carbohydrate reaching the hindgut. Let's break down the specific triggers.

Excess Starch and Sugar in the Diet

Horses have a limited capacity to digest starch in the small intestine. Research suggests the small intestine can only process around 1–2 grams of starch per kilogram of bodyweight per meal. When a horse receives more starch than this threshold — typically from large feeds of cereal grains like oats, barley, or maize — the excess passes undigested into the hindgut.

Once in the hindgut, this starch is rapidly fermented by bacteria, producing large amounts of lactic acid rather than the normal, gentler VFAs. Lactic acid drops the pH sharply and creates a hostile environment for fibre-fermenting microbes.

Large, Infrequent Concentrate Meals

Even if the total daily amount of grain isn't excessive, feeding it in one or two large meals can overwhelm the small intestine's digestive capacity. The sheer volume of starch arriving at once means more escapes into the hindgut. Splitting concentrates into smaller, more frequent meals significantly reduces this risk.

Sudden Dietary Changes

The microbial population in the hindgut is finely adapted to whatever the horse has been eating. A sudden switch — say, from a hay-based diet to a grain-heavy regimen, or even a rapid change in pasture quality — doesn't give the microbiome time to adjust. The result is microbial imbalance (dysbiosis) and a cascade of acid production.

Fructan-Rich Pasture

It's not only grain that causes problems. Lush pasture, particularly cool-season grasses in spring and autumn, can be rich in fructans — a type of storage carbohydrate that is poorly digested in the small intestine. Fructans pass to the hindgut where they are rapidly fermented, leading to the same acidotic chain of events. This is one reason laminitis cases spike in spring.

Stress and Intensive Management

While diet is the primary driver, factors like transport stress, intense exercise, prolonged stall confinement, and the use of certain medications (particularly NSAIDs) can compromise gut integrity and contribute to an acidic hindgut environment.

Symptoms of Hindgut Acidosis

One of the challenges with hindgut acidosis is that the symptoms can be subtle, especially in mild or chronic cases. It's often a "hidden" condition that owners attribute to other causes.

Digestive Signs

• Loose or acidic-smelling droppings — Faeces may be softer than normal, cow-pat-like, or have a distinctly sour smell.
• Intermittent mild colic — Recurring low-grade abdominal discomfort that resolves on its own.
• Excessive gas and bloating — Audible gut sounds or a distended abdomen after feeding.
• Reduced appetite — Particularly a reluctance to eat concentrates, or leaving food.
• Free faecal water — Liquid running down the hind legs before or after passing droppings.

Behavioural Changes

• Girthiness or sensitivity around the flanks — Often mistaken for a saddle-fit issue.
• Irritability or grumpiness — Especially around feeding time or when being tacked up.
• Reluctance to work or poor performance — The horse may feel "flat" or unwilling.
• Wood chewing or crib-biting — Sometimes an attempt to buffer stomach and gut acidity.

Wider Health Issues

• Laminitis — Bacterial die-off in the hindgut releases endotoxins that enter the bloodstream through a compromised gut wall. These endotoxins are strongly implicated in triggering laminitis.
• Poor hoof quality — Weak, crumbly, or slow-growing hooves can reflect chronic hindgut disturbance.
• Weight loss or poor condition — When fibre-fermenting microbes are compromised, the horse loses a major energy source.
• Dull coat and general lack of vitality — A struggling gut affects nutrient absorption across the board.
• Stereotypic behaviours — Weaving, box-walking, and other vices may have a gut-discomfort component.

If your horse shows several of these signs — especially loose droppings combined with behavioural changes or recurring hoof problems — hindgut acidosis should be on your list of potential causes.

How Hindgut Acidosis Differs from Gastric Ulcers

It's worth noting that hindgut acidosis is not the same as gastric (stomach) ulcers, although the two can occur simultaneously. Gastric ulcers involve acid damage to the stomach lining, whereas hindgut acidosis is an imbalance in the large intestine. Both conditions are driven by diet and management, and both are common in performance horses, but they require different approaches.

Some researchers now use the term "right dorsal colitis" or "colonic ulceration" to describe severe hindgut damage, which can be a consequence of prolonged acidosis.

Diagnosing Hindgut Acidosis

There is no single, simple diagnostic test for hindgut acidosis that is widely available in general practice. Vets may consider:

• Faecal pH testing — A low faecal pH can be suggestive, though results vary.
• Faecal blood tests — Detecting occult blood in manure may indicate hindgut mucosal damage.
• Clinical history and diet review — Often the most practical approach. A vet or nutritionist reviewing the horse's diet, management, and symptoms can build a strong picture.
• Response to dietary change — In many cases, a trial period of dietary modification that resolves symptoms is the most convincing evidence.

Dietary Solutions for Hindgut Acidosis

The good news is that hindgut acidosis responds very well to dietary management. In most cases, the right feeding changes will see significant improvement within two to four weeks.

1. Prioritise Forage

Forage — hay, haylage, or access to appropriate pasture — should form the foundation of every horse's diet. Aim for a minimum of 1.5% of bodyweight in forage daily (that's 7.5 kg for a 500 kg horse). Fibre fermentation in the hindgut produces gentle, beneficial VFAs and supports a healthy microbial population.

If your horse needs to gain weight or has higher energy demands, increase forage quantity or quality before reaching for the grain bucket.

2. Reduce Starch Intake

Keep total starch intake below 1 g per kg of bodyweight per meal, and ideally below 2 g per kg of bodyweight per day. In practical terms, this means:

• Limiting cereal grain-based feeds.
• Choosing feeds labelled as "low starch" or based on fibre and oil rather than cereals.
• Avoiding sweet feeds or mixes with visible whole grains.

For horses in hard work that need extra calories, consider replacing grain with high-fat sources like vegetable oil, rice bran, or copra meal, and super-fibres like beet pulp or soya hulls.

3. Feed Smaller, More Frequent Meals

If concentrates are necessary, split them into at least three or four small meals per day rather than one or two large ones. This gives the small intestine the best chance of digesting starch before it reaches the hindgut.

4. Make All Dietary Changes Gradually

Any change to your horse's diet — whether it's a new feed, different hay, or increased pasture access — should be introduced over a minimum of 7 to 14 days. This allows the hindgut microbiome to adapt.

5. Manage Pasture Access Carefully

During peak fructan periods (cool mornings in spring and autumn, or stressed grass during drought), consider limiting grazing time or using a grazing muzzle. Turnout on well-managed, mature pasture carries less risk than lush, fast-growing grass.

6. Consider Hindgut Buffers

Specific hindgut buffer supplements — often based on encapsulated sodium bicarbonate, calcium carbonate, or magnesium oxide designed to release in the hindgut — can help raise pH and support microbial recovery. These are not the same as stomach antacids and should be chosen specifically for hindgut support.

Live yeast supplements (particularly Saccharomyces cerevisiae) have also shown promise in stabilising hindgut pH and supporting fibre-fermenting bacteria.

7. Support Gut Wall Integrity

Supplements containing ingredients like pectin, lecithin, glutamine, or short-chain fatty acid precursors may support the repair of a compromised gut lining. While the evidence base varies, many horse owners and vets report positive results when these are used alongside dietary correction.

8. Review the Whole Diet

It's easy to focus on one element — say, swapping to a low-starch feed — while overlooking other sources of excess sugar or starch in the overall ration. Treats, chaff, supplements mixed with molasses, and even certain hay batches can contribute more than you think.

Analysing your horse's full diet is one of the most effective steps you can take. A thorough nutritional analysis helps you see exactly where starch, sugar, and fibre levels sit, so you can make targeted changes rather than guessing.

A Sample Recovery Plan

Here's what a practical dietary adjustment might look like for a 500 kg horse with suspected hindgut acidosis:

Monitor droppings, behaviour, and overall condition closely. Most owners notice firmer droppings and improved demeanour within two to three weeks.

When to Call the Vet

While mild hindgut acidosis is very manageable through diet, you should involve your vet if:

• Your horse shows signs of acute laminitis (shifting weight, increased digital pulses, reluctance to move).
• Colic episodes are frequent or severe.
• There is blood or mucus in the droppings.
• Weight loss is rapid or unexplained.
• Symptoms do not improve after four weeks of dietary changes.

Severe hindgut acidosis can lead to endotoxaemia and systemic illness, so don't hesitate to seek veterinary advice if you're concerned.

Prevention Is Better Than Cure

Once you've resolved a bout of hindgut acidosis, the focus should shift to prevention. The principles are straightforward:

• Forage first, always. Build the diet around fibre.
• Keep starch low and split it up. If your horse needs grain, keep meals small.
• Change feeds gradually. Give the microbiome time to adjust.
• Watch the grass. Manage pasture access during high-risk periods.
• Monitor your horse. Droppings, behaviour, and hoof health are your daily report card on gut function.

Hindgut acidosis is one of those conditions that, once you understand it, you'll start to see it everywhere — in the girthy horse, the one with persistent soft droppings, or the horse that just isn't quite right. The reassuring truth is that thoughtful dietary management can resolve and prevent it, keeping your horse's hindgut — and the trillions of microbes that call it home — in good working order.