Digestion

Hindgut Acidosis in Horses: Signs, Causes & Management

Hindgut acidosis is widely discussed in performance-horse nutrition, but it is often oversimplified. Understanding it means understanding what reaches the hindgut, how microbes ferment it, why starch and pasture carbohydrates matter, and what owners can realistically do through feeding and management.

Written by Ben Nedas Reviewed by Dr Rosemary Waring

Quick answer

What horse owners need to know

Hindgut acidosis occurs when the cecum and colon become more acidic than normal, commonly because rapidly fermentable carbohydrate reaches the hindgut in excessive amounts.

A major pathway is undigested starch escaping small-intestinal digestion. Hindgut microbes rapidly ferment that starch, increasing acid production and changing the microbial environment. Fructans from pasture can also contribute to rapid fermentation.

Possible signs include loose manure, increased gas, poor condition, changes in demeanor and reduced performance, but these are not specific enough to diagnose hindgut acidosis.

Management starts with forage, sensible concentrate meal sizes, gradual diet changes and careful pasture management. Horses showing persistent digestive problems or signs of illness should be assessed by a veterinarian.

Start with the horse

Understanding what you're seeing

What you may notice

Hindgut acidosis does not have one unique outward sign, and many horses may show only subtle or nonspecific changes.

Owners may report:

• Loose or inconsistent manure
• Increased gas or flatulence
• A bloated appearance
• Changes in appetite
• Difficulty maintaining weight or condition
• Reduced performance or stamina
• A change in demeanor or willingness to work
• Increased sensitivity or irritability
• Recurrent digestive discomfort
• Colic episodes in more significant cases

None of these observations proves that a horse has hindgut acidosis.

The same signs can occur with many other feeding, veterinary, dental, behavioral or management problems. Their value is in prompting a broader review of the horse rather than providing a diagnosis by themselves.

Common causes

Hindgut acidosis is fundamentally associated with excessive production or accumulation of acid within the cecum and colon.

Common dietary and management factors that can contribute include:

• Large concentrate or grain meals
• High dietary starch intake
• Starch that is relatively poorly digested in the small intestine
• Sudden access to large quantities of grain
• Rapid dietary changes
• Rapid increases in concentrate feeding
• Inadequate forage relative to concentrate intake
• Sudden pasture changes
• High intake of rapidly fermentable pasture carbohydrates such as fructans
• Feeding practices that create large fluctuations in substrate entering the hindgut

The problem is not that starch itself is inherently bad for horses. Starch is an important energy source, particularly in athletic horses.

The problem arises when more rapidly fermentable carbohydrate reaches the hindgut than the microbial ecosystem can handle without a substantial change in fermentation and pH.

What should you check first?

Start with the ration rather than the supplement shelf.

1. How much concentrate is being fed per meal?
Look at both the weight of the feed and its starch percentage. Two feeds of the same weight can deliver very different starch loads.

2. How many meals is the daily concentrate divided into?
A smaller starch load supplied more frequently reduces the amount presented to the small intestine at one time.

3. What is the starch source?
Starch digestibility differs between cereals and is affected by processing. Oat starch, for example, is generally more readily digested before the hindgut than untreated corn or barley starch.

4. How much forage does the horse receive?
The hindgut evolved around continuous microbial fermentation of fibrous plant material. Restricted forage and high-concentrate feeding move the ration away from that pattern.

5. Has anything changed recently?
New hay, increased grain, a new competition ration, reduced turnout or sudden pasture access can all change what reaches the hindgut.

6. What is happening with the grass?
Pasture carbohydrate composition changes with species, season, weather and growing conditions.

7. What does the manure look like?
Loose or inconsistent manure and increased gas can provide clues that digestive fermentation has changed, but neither is diagnostic of hindgut acidosis.

8. Has anything else changed?
Track appetite, body condition, behavior, performance, abdominal comfort and any previous colic history.

Looking at these factors together is much more useful than trying to diagnose hindgut health from one sign in isolation.

Feeding & management steps

The foundation of hindgut management is feeding practice.

Useful principles include:

• Make forage the foundation of the ration wherever possible
• Avoid unnecessarily large concentrate meals
• Split required concentrate intake across multiple smaller meals
• Review the amount of starch supplied by each meal, not simply the weight of the feed
• Consider the digestibility and processing of the starch source
• Introduce new feeds and forage gradually
• Avoid sudden increases in grain or concentrate intake
• Maintain consistent access to clean, fresh water
• Manage access to rapidly changing pasture appropriately
• Avoid long periods without forage where possible
• Review the ration when workload changes rather than simply adding more grain
• Consider fiber- and fat-based energy sources where appropriate
• Keep dental care up to date so forage can be chewed effectively

Performance horses may genuinely require greater dietary energy. The goal is not necessarily to remove starch completely, but to supply energy without overwhelming the horse’s capacity to digest starch before it reaches the hindgut.

Where hindgut problems are suspected, involve your veterinarian and an appropriately qualified equine nutrition professional when reviewing the ration.

When to contact your veterinarian

The signs sometimes associated with hindgut acidosis are not specific enough to diagnose the condition yourself.

Contact your veterinarian promptly if your horse shows signs of colic, develops profuse watery diarrhea, stops eating, has a fever, appears depressed or weak, becomes dehydrated, or develops sudden foot pain or signs consistent with laminitis.

Veterinary advice is also appropriate for persistent loose manure, recurrent abdominal discomfort, unexplained weight loss, recurring appetite changes or a significant drop in performance.

Hindgut acidosis can overlap with many other gastrointestinal, metabolic and management problems. A horse displaying persistent signs deserves investigation rather than assuming acidity is the underlying cause and treating it solely through supplements or dietary changes.

The digestive connection

Where digestion fits

The horse is designed to digest different parts of its diet in different places.

Starch is primarily intended to be broken down enzymatically before it reaches the hindgut. Fiber, by contrast, is fermented by the enormous microbial population living within the cecum and colon.

Problems can arise when the amount of starch entering the small intestine exceeds the horse’s capacity to digest it efficiently. The undigested starch then reaches the hindgut and becomes a highly fermentable substrate for bacteria.

Rapid fermentation increases acid production. As pH falls, the microbial population can change: acid-tolerant organisms become more competitive while some of the bacteria responsible for normal fiber fermentation are disadvantaged.

This can reduce digestive stability and, in more substantial carbohydrate-overload situations, contribute to serious gastrointestinal and metabolic consequences.

Fructans from pasture provide another route by which rapidly fermentable carbohydrate can enter the hindgut.

This means hindgut health is influenced not only by the bacteria already living there, but also by what the horse’s feeding and digestive processes deliver to them.

Supporting effective upstream digestion is therefore one part of managing the downstream environment.

EquiNectar research

A measurable change in the hindgut fermentation environment

EquiNectar research has directly measured a change in fecal pH in horses maintained within demanding performance feeding programs.

Seventy-eight Thoroughbred racehorses were recruited across nine UK training yards. EquiNectar was added to their existing feeds while their normal feeding and training programs continued.

Over four weeks, mean fecal pH increased from 6.20 to 6.40, with a reported statistical result of p<0.0001.

The proportion of horses above the study’s stated normal fecal-pH threshold increased from 11% at the beginning of the study to 55% after four weeks.

This is relevant because fecal pH provides one measurable window into the fermentation environment of the hindgut. Other equine research has linked concentrate feeding and incomplete small-intestinal starch digestion with lower hindgut and fecal pH.

The EquiNectar result therefore provides direct evidence that supplementation was associated with a measurable downstream digestive change while the horses remained within their normal feeding programs.

78 horses Thoroughbred racehorses recruited across nine UK training yards

6.20 → 6.40 Mean fecal pH over four weeks

11% → 55% Above the study’s stated normal fecal-pH threshold after four weeks

What this research does — and doesn't — tell us

The EquiNectar racehorse study measured fecal pH rather than directly measuring pH within the cecum or colon.

Fecal pH provides a useful and practical window into changes in the hindgut fermentation environment, but it is an indirect measure and is not a definitive diagnostic test for hindgut acidosis.

The study followed Thoroughbred racehorses under normal training and feeding conditions and did not include a randomized placebo-control group. It therefore demonstrates an association between EquiNectar use and a measurable change in fecal pH over time, but it cannot by itself establish that EquiNectar prevents or treats hindgut acidosis.

The findings should also not be assumed to predict the response of every individual horse or every type of diet.

EquiNectar is a nutritional digestive supplement, not a veterinary treatment. Horses with persistent digestive problems, recurrent colic, significant weight loss, diarrhea or other signs of illness should be assessed by a veterinarian.

Where EquiNectar fits

Supporting digestion as part of good management

EquiNectar is designed to support digestion rather than treat hindgut acidosis.

Its Enzyme Rich Malt Extract provides naturally occurring digestive enzyme activities, including amylase and fructanase, which act on components of the horse’s normal diet.

The principle is upstream digestion. When more dietary carbohydrate is broken down earlier in the digestive tract, less undigested substrate is available for rapid fermentation in the hindgut.

This is particularly relevant to horses receiving concentrate feeds, performance diets or variable pasture, but EquiNectar should form part of a wider management approach rather than replace good feeding practice.

Adequate forage, sensible concentrate meal sizes, gradual dietary changes, appropriate pasture management, access to water, dental care and veterinary investigation where necessary remain fundamental to digestive health.

Explore EquiNectar

Common questions

Frequently asked questions

What is hindgut acidosis in horses?

Hindgut acidosis occurs when the environment within the cecum and colon becomes more acidic than normal. A common dietary pathway is excessive rapidly fermentable carbohydrate reaching the hindgut, where microbes ferment it and produce acids. This can alter the microbial population and interfere with the normal fermentation of fiber. Severe or sudden carbohydrate overload can have serious consequences, while more moderate changes may be less obvious.

What causes hindgut acidosis in horses?

What are the signs of hindgut acidosis in horses?

There is no single set of signs that confirms hindgut acidosis. Possible observations can include loose or inconsistent manure, increased gas, reduced appetite, difficulty maintaining condition, changes in demeanor, abdominal discomfort or reduced performance. These signs are nonspecific and can have many other causes, so they should not be used to diagnose hindgut acidosis on their own.

Can grass contribute to hindgut acidosis?

Yes. Cool-season grasses can contain substantial amounts of fructans, particularly under certain growing and weather conditions. Fructans are fermented by microbes in the hindgut, and a large or sudden carbohydrate load can alter fermentation and lower pH. This is one reason gradual pasture introduction and appropriate grazing management are important, particularly during periods of rapid grass growth or changing conditions.

How much starch should a horse have in one meal?

A commonly cited upper guideline is no more than approximately 2 g of starch per kilogram of body weight per meal, although the amount a horse can digest also depends on the grain source, processing, meal composition and the individual horse. For a 500 kg horse, 2 g/kg represents 1 kg of starch — not 1 kg of concentrate feed. Many horses can be managed comfortably below this level by splitting concentrate intake into smaller meals and relying more heavily on forage and fiber-based energy sources.

How is hindgut acidosis diagnosed?

Directly measuring pH within the cecum or colon is not practical in routine horse management, so hindgut acidosis can be difficult to diagnose definitively. Veterinarians and nutritionists may consider diet, clinical history and other digestive findings. Fecal pH can provide useful information about changes in the fermentation environment, but it is an indirect measure and should not be treated as a definitive diagnosis of hindgut acidosis.

Can EquiNectar help support horses at risk of hindgut acidosis?

EquiNectar is a digestive supplement rather than a treatment for hindgut acidosis. It provides naturally occurring digestive enzyme activities that help break down dietary components earlier in digestion. This approach is particularly relevant to starch: improving upstream digestion can reduce the amount of undigested substrate available for rapid fermentation downstream. In an EquiNectar study involving 78 Thoroughbred racehorses, mean fecal pH increased from 6.20 to 6.40 over four weeks while the horses remained within their normal feeding and training programs. Fecal pH is an indirect marker, so this should not be interpreted as proof that EquiNectar treats or prevents hindgut acidosis.