Does High-Starch Feed Make Horses Hot?

By Dr Duncan Houston

A lot of horse owners say the same thing: “Protein makes my horse hot.”

It is one of the most common feeding myths in horses. The more accurate version is this: protein is rarely the problem. Excess digestible energy, especially from starch-heavy grain or concentrate meals, is much more likely to affect reactivity in some horses.

That does not mean every sharp, spooky, anxious, or reactive horse is being fed too much starch. Behaviour is never that simple. Pain, saddle fit, lameness, ulcers, workload, turnout, training, temperament, hormones, and stress all matter too. But if a horse becomes more reactive after a grain increase, a new sweet feed, or larger concentrate meals, the starch level deserves a very close look.

Quick Answer

High-starch feeds can make some horses more reactive, especially when large grain or concentrate meals replace forage. Research comparing high-starch and high-fibre diets found horses on high-starch diets had higher heart rates during handling and more interrupted eating when exposed to unfamiliar stimuli. Protein does not cause excitable behaviour in horses, and energy mainly comes from carbohydrates and fat rather than protein. (SRUC, Scotland's Rural College)

What Does “Hot” Mean in Horses?

When owners describe a horse as “hot,” they usually mean the horse feels more reactive, tense, spooky, anxious, forward, distracted, difficult to settle, or explosive under saddle.

Common descriptions include:

• More spooking

• More rushing

• Harder to focus

• Increased jig-jogging

• More tension during handling

• Stronger reaction to noise or movement

• More anxious in new environments

• More difficult to settle after work

• More reactive during feeding, grooming, or saddling

The problem is that “hot” is not a diagnosis. It is an observation.

A horse can be hot because it is overfed. It can also be hot because it is sore, stressed, underworked, overworked, badly fitted, socially unsettled, ulcer-prone, or simply a sharper temperament. The feed bucket is a common culprit, but it is not the only suspect.

The Protein Myth

Protein gets blamed because people see a number on the feed bag and assume higher protein means more energy. That is not how it usually works in horses.

Merck Veterinary Manual states directly that protein does not cause excitable behaviour in horses. Energy comes mainly from carbohydrates and fat, while dietary protein is normally not used as a major energy source and contributes very little to overall energy production. (Merck Veterinary Manual)

Protein is still important. Horses need it for:

• Muscle repair

• Growth

• Skin and coat

• Hoof quality

• Pregnancy

• Lactation

• Immune function

• Tissue repair

But if a horse becomes sharper after a feed change, look at the calories, starch, sugar, meal size, and workload match before blaming the crude protein percentage.

What Is Starch?

Starch is a nonstructural carbohydrate found in higher levels in cereal grains such as oats, barley, corn, and many grain-based concentrates.

Starch is useful because it provides digestible energy. Performance horses, hard keepers, lactating mares, growing horses, and horses in heavier work may need more energy than forage alone can provide.

The issue is not that starch is evil. The issue is that too much starch, too quickly, or in large meals can overwhelm the digestive system and may affect behaviour, gut health, insulin response, colic risk, and laminitis risk in susceptible horses.

In practice, starch becomes more suspicious when:

• The horse is eating large grain meals

• Forage intake has dropped

• The horse changed feed recently

• Sweet feed or cereal grain has been increased

• The horse is not doing enough work for the calories fed

• Behaviour changed after the diet changed

• The horse is also showing loose manure, ulcers, girthiness, colic signs, or laminitis risk

What Does the Research Say?

A 2015 study compared horses on high-starch and high-fibre diets using handling and novel stimulus tests. Horses fed the high-starch diet had significantly higher average and maximum heart rates during handling, and showed more interrupted eating during novel stimulus exposure compared with the high-fibre diet. The authors concluded that high-starch feeding had some effect on reactivity and handling behaviour, especially heart rate reactivity. (SRUC, Scotland's Rural College)

A later Scientific Reports study found that high-starch diets altered equine faecal microbiota and increased behavioural reactivity in ponies. The study reported that ponies were more reactive and less settled when fed a high-starch diet compared with a high-fibre diet. (Nature)

The practical takeaway is not “every hot horse needs zero starch.” It is more precise than that:

High-starch diets can increase reactivity in some horses, especially when they replace forage and create larger glycaemic or hindgut effects.

That matters because many owners increase grain to create more energy for work, then wonder why the horse feels less rideable.

Why Horses Usually Do Better With Forage First

Horses are designed to eat forage for long periods across the day. Their digestive system is built around steady fibre intake, not large, infrequent starch-heavy meals.

Merck Veterinary Manual recommends that most healthy horses have free access to hay or pasture forage, salt, and fresh water. It also notes that many horses on good-quality forage need little or no concentrate supplementation unless they are in hard work, pregnant, lactating, struggling to maintain weight, or have limited forage access. (Merck Veterinary Manual)

Forage helps support:

• Normal chewing time

• Saliva production

• Gastric buffering

• Hindgut microbial stability

• More stable energy intake

• Normal manure consistency

• Reduced boredom

• Reduced feeding-related frustration

When forage drops and grain rises, the horse may receive more fast energy but less digestive stability. That is when you can see a horse become sharper, more unsettled, more ulcer-prone, or more reactive.

How Much Grain Is Too Much?

This depends on the horse, feed type, workload, body condition, metabolic risk, and forage intake.

A useful safety benchmark is meal size.

Merck Veterinary Manual states that horses should not receive more than 0.5% of body weight in grain-based concentrates in a single feeding. For a 500 kg horse, that equals about 2.5 kg, or 5.5 lb, in one meal. Feeding more than this in one meal increases the risk of laminitis and digestive upset. (Merck Veterinary Manual)

Merck also notes that larger grain meals, more than 0.25% of body weight, should not be offered less than 1 hour before strenuous exercise, transport, or other stress. (Merck Veterinary Manual)

A practical owner rule:

If your horse needs a lot of concentrate, split it into smaller meals, keep forage high, and review whether a lower-starch, higher-fibre, or higher-fat feed would be safer.

Why High-Starch Meals Can Affect the Gut

The horse’s small intestine can digest starch, but there is a limit. If too much starch reaches the hindgut, it can disrupt the microbial environment. That can contribute to gas, acidity, loose manure, discomfort, and increased risk of colic or laminitis in some horses.

Merck Veterinary Manual states that feeding more than 50% of the ration dry matter as high starch or sugar concentrates has been documented to increase the risk of laminitis, colic, and equine gastric ulcer syndrome. (Merck Veterinary Manual)

This is where behaviour and gut health overlap. A horse with hindgut upset, ulcers, or discomfort may look “hot,” but the behaviour may actually be pain, anxiety, or digestive irritation.

A horse is not being naughty because the feed was delicious. The gut may be sending a strongly worded complaint.

Fat as an Alternative Energy Source

Fat can be useful when a horse needs more calories but does not tolerate starch-heavy feeds well.

Fat provides dense energy without the same starch load. University of Georgia Extension notes that fat is an excellent and easily digestible energy source for horses, and that many commercial feeds include added fat, often in the 6 to 12% range. It also warns that when fat increases energy density, the rest of the diet still needs enough protein, vitamins, and minerals. (CAES Field Report)

Fat sources may include:

• Stabilised rice bran

• Ground flaxseed

• Vegetable oil

• Commercial high-fat, low-starch feeds

• Fat-added performance feeds

But fat is not magic either.

It should be introduced gradually, balanced properly, and used for the right horse. A fat-added diet may help reduce reliance on starch, but it will not fix pain, poor training, ulcers, or excess calories.

Which Horses Need Extra Care With Starch?

Laminitis-Prone Horses

Horses with a history of laminitis need careful sugar and starch control. Diet changes should be veterinary-guided.

Horses With Equine Metabolic Syndrome

Equine metabolic syndrome involves insulin dysregulation and increased laminitis risk. Merck states that diet is the most important part of EMS management and that grain and treats should be eliminated for EMS patients, with low-nonstructural-carbohydrate hay used as the foundation. (Merck Veterinary Manual)

Easy Keepers and Ponies

Ponies, donkeys, native breeds, Arabians, and mustangs can be very efficient with calories. UC Davis notes that EMS often affects thrifty equids and that high-carbohydrate feeds can trigger higher insulin responses in affected horses. (ceh.vetmed.ucdavis.edu)

Horses With Gastric Ulcers

High-concentrate, low-forage diets can increase ulcer risk. If a horse is hot, girthy, reactive, poor in appetite, or worse during work, ulcers should be on the list.

Horses in Light Work

Many horses in light work do not need large concentrate meals. If the work level is low and the feed bucket is high, behaviour and weight can both become problems.

Performance Horses

Performance horses may need more energy, but not always more starch. For some, a higher-fibre, higher-fat ration gives better rideability than simply adding more cereal grain.

Severity and Risk Framework

The key decision point is simple:

A sharp horse with no other signs may need a feed and management review. A sharp horse with pain, colic signs, laminitis signs, poor appetite, or sudden behaviour change needs a vet.

What Else Can Make a Horse Hot or Reactive?

Do not blame starch until you have considered the other common causes.

Pain

Pain is one of the biggest causes of reactive behaviour. Hocks, stifles, feet, back, neck, sacroiliac region, teeth, and soft tissue injuries can all make a horse feel explosive.

Saddle Fit

A badly fitted saddle can make a horse rush, buck, hollow, resist canter, pin the ears, or become reactive during girthing.

Gastric Ulcers

Ulcers can cause girthiness, sensitivity, poor appetite, dull coat, irritability, poor performance, and anxiety around work or feeding.

Under-Exercise

A horse receiving performance-level calories but only light work may simply have more energy than the job requires.

Overtraining

Tired horses can also become reactive. Fatigue, muscle soreness, and mental stress can look like excitability.

Turnout and Management

Limited turnout, isolation, herd stress, too much stabling, poor routine, or lack of movement can make some horses sharper.

Training Conflict

A confused horse can become tense. If the aids are inconsistent, the work is too difficult, or the horse is punished for being anxious, the behaviour can escalate.

Hormones

Mares may show cycle-linked behaviour. Geldings and stallions can also show management or social behaviour that looks like feed reactivity.

Medical Problems

Vision issues, neurologic disease, tying-up disorders, respiratory disease, endocrine disease, and systemic illness can all change behaviour.

The feed bucket matters, but it is not allowed to become the scapegoat for everything.

When Is This an Emergency?

Diet-related reactivity itself is not usually an emergency. But some signs mean the issue is no longer just behaviour.

Call your vet urgently if your horse has:

• Colic signs

• Repeated pawing, rolling, or looking at the flank

• Sweating with abdominal pain

• Refusing feed

• Choke signs

• Feed or saliva coming from the nose

• Sudden severe diarrhoea

• Laminitis signs

• Rocked-back stance

• Heat or strong digital pulses in the feet

• Sudden severe lameness

• Collapse

• Tremors, weakness, or incoordination

• Sudden dangerous behaviour change

• Grain-bin break-in or suspected concentrate overload

A horse that breaks into the feed room and eats a large amount of grain should be treated as urgent, even if it looks fine initially. Hindgut upset and laminitis risk can follow later.

What Should You Do Next?

1. Write Down the Current Diet

Record exactly what the horse eats each day:

• Hay type and amount

• Pasture access

• Grain or concentrate type

• Amount per meal by weight, not scoops

• Number of meals per day

• Supplements

• Oil or fat source

• Treats

• Workload

• Body condition score

A “scoop” is not a feed plan. Weigh it.

2. Check Whether the Behaviour Changed After the Feed Changed

This is one of the most useful clues.

Ask:

• Did the horse become hotter after adding grain?

• Did it start after changing brands?

• Did it worsen when pasture changed?

• Did it improve when feed was reduced?

• Does it happen after meals?

• Is it worse on days off?

• Is it worse in cold weather when work drops?

Patterns matter.

3. Do Not Suddenly Stop Everything

Abrupt feed changes can upset the gut. Unless there is an emergency such as grain overload, mouldy feed, colic, or laminitis, adjust the diet gradually over 7 to 14 days.

4. Increase Forage Consistency

Most horses do better when forage is the foundation. Ensure the horse has enough good-quality forage before adding more concentrate.

5. Reduce Starch Before Blaming Protein

If the horse is reactive, review the starch and sugar load, not just crude protein.

Useful changes may include:

• Reducing grain amount

• Splitting concentrate into smaller meals

• Switching from sweet feed to a lower-starch feed

• Using a ration balancer for nutrients without excess calories

• Adding fat or fibre-based calories if weight is needed

• Increasing hay quality rather than grain amount

• Matching calories to workload

6. Review Workload

A horse fed like an eventer but worked like a weekend lawn ornament may feel like a kite with legs.

Feed for the work being done, not the work you planned to do in an optimistic mood.

7. Rule Out Pain

If the behaviour is sudden, severe, dangerous, or not clearly linked to feed, get the horse checked.

A diet change will not fix sore hocks, ulcers, back pain, dental pain, or a bad saddle.

8. Track Behaviour for 2 to 4 Weeks

Use a simple log:

• Feed

• Work

• Turnout

• Behaviour score

• Spooking

• Sweating

• Manure

• Appetite

• Girthing reaction

• Pain signs

This turns vague “he feels hot” into useful clinical information.

9. Get Help With Metabolic Horses

If your horse has laminitis, EMS, PPID, obesity, cresty neck, or insulin dysregulation, do not experiment with high-starch feeds. Work with your vet or nutritionist. EMS management relies heavily on diet, forage control, exercise where safe, and low-carbohydrate feeding. (Merck Veterinary Manual)

10. Reassess After the Diet Change

If the horse improves, starch or energy level was likely part of the problem. If there is no improvement, keep looking.

A failed feed trial is still useful. It tells you the answer may be pain, training, ulcers, workload, turnout, or another medical issue.

Common Mistakes Owners Make

Blaming Protein

Protein is not the usual reason horses become hot. Starch, sugar, total calories, workload mismatch, pain, and stress are much more likely. (Merck Veterinary Manual)

Feeding for Work the Horse Is Not Doing

Feed should match actual workload, not ambition, weather-dependent plans, or “he might need more energy.”

Changing Feed Too Fast

Sudden changes can upset the gut. Make changes gradually unless there is a medical reason to stop immediately.

Feeding Large Grain Meals

Large grain meals increase digestive risk. Merck recommends keeping grain-based concentrates below 0.5% of body weight per meal. (Merck Veterinary Manual)

Forgetting Forage

Forage is not filler. It is the foundation of equine gut health.

Adding Fat Without Balancing the Diet

Fat adds calories, but the diet still needs adequate protein, vitamins, and minerals. (CAES Field Report)

Ignoring Pain

A horse that is reactive because of pain will not become comfortable just because the starch was reduced.

Prevention: A Better Feeding Plan for Reactive Horses

A good prevention plan includes:

• Forage as the foundation

• Concentrate only when needed

• Feed amounts weighed, not guessed

• Small concentrate meals

• Gradual feed changes

• Lower-starch options for reactive horses

• Ration balancers for horses needing nutrients without extra calories

• Fat or fibre-based calories for horses needing weight

• Regular body condition scoring

• Adequate turnout and movement

• Consistent workload

• Saddle, dental, hoof, and lameness checks

• Ulcer assessment if signs fit

• Special low-starch plans for EMS or laminitis-prone horses

The best feeding plan should make the horse healthier and easier to manage. If the feed gives the horse more calories but makes it harder to ride, harder to handle, and harder to keep sound, it is not the right plan.

FAQ

Does protein make horses hot?

No, protein is not the usual cause of excitable behaviour in horses. Merck Veterinary Manual states that protein does not cause excitable behaviour, and that energy mainly comes from carbohydrates and fat rather than protein. (Merck Veterinary Manual)

Can grain make a horse more reactive?

Yes, some horses become more reactive on high-starch grain or concentrate diets. Research has found higher heart rates and more reactive behaviour in horses or ponies fed high-starch diets compared with high-fibre diets. (SRUC, Scotland's Rural College)

Should I remove all grain from a hot horse?

Not always. Some horses need concentrate calories for workload or body condition. The smarter approach is to review forage, total calories, starch level, meal size, workload, and medical issues, then reduce starch gradually if appropriate.

Is fat better than starch for reactive horses?

Fat can be a useful alternative calorie source because it adds energy without the same starch load. It should be introduced gradually and balanced with the rest of the ration. (CAES Field Report)

When should I call a vet?

Call a vet if the behaviour change is sudden, dangerous, associated with pain, colic signs, laminitis signs, poor appetite, weight loss, diarrhoea, choke, or suspected grain overload. Diet may be involved, but those signs need proper veterinary assessment.

Final Thoughts

The old idea that protein makes horses hot needs to be retired.

If a horse becomes sharper after a feed change, the better suspects are starch, sugar, total calorie intake, meal size, workload mismatch, gut discomfort, pain, or stress. High-starch diets can increase reactivity in some horses, and large concentrate meals can also increase digestive and laminitis risk.

The best plan is not to starve the horse or blame one number on the feed bag. Build the diet around forage, match calories to work, keep starch sensible, use fat or fibre-based calories where appropriate, and investigate pain if the behaviour does not clearly follow the feed.

A reactive horse is trying to tell you something. The feed bucket is one place to look, but it is not the only one.

If your horse has become hotter, more reactive, difficult to ride, or unsettled after a feed change, ASK A VET™ can help you work through the diet, urgency, and when veterinary assessment is needed.