How to Change Your Horse’s Diet Safely Without Causing Colic or Laminitis

By Dr Duncan Houston

A new feed may look healthier, a new hay batch may smell fresher, and spring pasture may seem like the most natural food possible.

The horse’s digestive system does not care that the change was well intentioned. It only knows that the ingredients arriving in the hindgut are suddenly different.

Abrupt changes in hay, pasture, concentrate or meal size can alter normal fermentation and increase the risk of diarrhoea, colic and, in susceptible horses, laminitis. The safest approach is to change one part of the ration at a time and give the digestive system time to adapt.

Quick Answer

Most meaningful changes in hay or concentrate should be introduced over at least 7 to 14 days. Replace approximately 20% to 25% of the previous feed every two to three days while monitoring appetite, manure, water intake and hoof comfort.

Pasture also counts as a major diet change. Start with brief grazing periods and increase access gradually, using a substantially more cautious plan for ponies, easy keepers and horses with insulin dysregulation or previous laminitis. (University of Minnesota Extension)

Why Can a Sudden Feed Change Be Dangerous?

Horses rely on a large and complex population of bacteria, fungi, protozoa and other microorganisms within the caecum and colon.

These microorganisms ferment fibre and other dietary material, producing energy sources that the horse can absorb. Different feeds support different microbial populations.

When the diet changes abruptly:

• Some organisms suddenly receive more substrate.

• Other organisms receive less.

• Fermentation patterns change.

• Gas and acid production may change.

• Manure consistency may alter.

• The horse may become less efficient at digesting the new ration during the transition.

This does not mean every abrupt feed change automatically causes microbial death, endotoxaemia or laminitis. However, rapid microbiome changes have been demonstrated after switches between conserved forage, hay and pasture, and the first several days appear to be an important period of adjustment. (PLOS)

What Did the New Zealand Study Actually Find?

A frequently quoted New Zealand study involved six yearling fillies changed abruptly from an ensiled forage and grain diet to pasture, while another six remained on pasture.

The faecal bacterial community of the changed group became broadly similar to the pasture group within four days. This showed that the microbial population responded rapidly to the new diet.

It did not prove that four days is a complete or universally safe transition period. The study involved a small group of young horses and a primarily forage-based dietary change. A later pony study also found measurable microbial changes during the first few days after abrupt grass-to-hay and hay-to-grass transitions. (PLOS)

The practical conclusion is:

The hindgut responds within days, but that is exactly why the transition should be controlled rather than abrupt.

What Counts as a Diet Change?

Owners often think only of changing the brand of grain. In practice, all of the following count:

• Changing from one hay species to another

• Opening a new batch of the same named hay

• Switching between cuttings

• Introducing alfalfa or lucerne

• Changing from dry hay to haylage

• Moving from hay to pasture

• Moving from pasture to hay

• Changing concentrate brands

• Changing from pellets to textured feed

• Increasing or decreasing the amount of grain

• Adding beet pulp, rice bran or oil

• Starting a high-fat feed

• Adding a large supplement meal

• Changing feeding frequency

• Allowing access to a new paddock

• Moving yards where the water, forage and routine all change

Recent changes in feed and housing are among the better-supported management-related risk factors for colic. The evidence does not show that feed changes cause every colic, but it supports taking changes in hay, concentrate, pasture and routine seriously. (PMC)

How Long Should a Feed Transition Take?

For most healthy adult horses, allow approximately 7 to 14 days.

A useful plan is to replace 20% to 25% of the old ration every two to three days. University of Minnesota guidance specifically recommends replacing only 20% to 25% every other day when changing hay or grain. (University of Minnesota Extension)

Transition stage	Previous feed	New feed
Days 1 to 3	75%	25%
Days 4 to 6	50%	50%
Days 7 to 9	25%	75%
Days 10 to 14	0%	100%

This is a framework, not a race.

Use a slower transition, sometimes two to three weeks, when the horse:

• Has previously had colic

• Has recurrent diarrhoea

• Has a history of laminitis

• Has insulin dysregulation

• Is elderly

• Has poor dentition

• Has recently travelled

• Is recovering from illness

• Is changing between very different diets

• Is moving from dry hay onto lush pasture

If appetite, manure or comfort deteriorates, stop increasing the new feed. Return to the last well-tolerated stage and reassess the horse.

Change One Major Component at a Time

Where possible, avoid changing the hay, concentrate, supplements and pasture in the same week.

A safer order is:

1. Establish consistent hay and water.

2. Transition the hay if necessary.

3. Allow appetite and manure to remain normal for several days.

4. Transition the concentrate.

5. Introduce pasture separately.

6. Add only supplements that have a defined purpose.

This makes it much easier to identify the cause if the horse develops loose manure, feed refusal, colic or hoof soreness.

Changing everything at once creates a digestive mystery with far too many suspects.

How Should You Change Hay?

Hay should be changed by weight, not by flake.

Two flakes from different bales may contain very different amounts of forage. Even two hay batches described as “the same grass hay” can differ in:

• Plant species

• Maturity

• Fibre

• Protein

• Sugar and starch

• Digestible energy

• Mineral content

• Dust

• Mould contamination

Mix the old and new hay physically where possible. If the horse sorts one type from the other, use several small feeding stations or alternate small amounts throughout the day while preserving the overall transition ratio.

Monitor:

• Appetite

• Amount of hay left

• Eating speed

• Manure quantity

• Manure moisture

• Abdominal comfort

• Coughing

• Water intake

A change in hay batch has been associated with increased colic risk in observational research, even when owners may consider it a minor change. (PMC)

What If the Old Hay Has Run Out?

Sometimes a gradual transition is impossible.

If the previous hay is unavailable:

• Choose the most similar clean forage possible.

• Begin with several smaller portions.

• Avoid introducing new concentrate or pasture simultaneously.

• Maintain unrestricted clean water.

• Monitor the horse closely for at least the next several days.

• Contact your veterinarian early if appetite, manure or comfort changes.

Do not use mouldy, spoiled or contaminated old hay simply to complete a transition. Hygienic safety takes priority.

How Should You Change Concentrate?

First determine whether concentrate is genuinely required.

Many healthy adult horses at maintenance or in light work can meet their energy needs through suitable forage, with a ration balancer used where vitamins, minerals or amino acids are lacking.

When changing concentrate:

1. Weigh the current meal.

2. Record the exact product and amount.

3. Weigh the new product rather than using the same scoop volume.

4. Replace 20% to 25% at a time.

5. Divide larger daily quantities into several meals.

6. Monitor the horse before increasing further.

Different feeds can have dramatically different densities. One scoop of a light high-fibre pellet may weigh far less than the same scoop filled with corn or another dense cereal feed.

How Large Can a Concentrate Meal Be?

A horse should not receive more than 0.5% of body weight in grain-based concentrate in one meal.

For a 500-kilogram horse, that is an upper limit of approximately 2.5 kilograms per meal. Larger daily amounts should be divided into smaller meals at least four hours apart. This is a maximum ceiling, not an ideal target. (Merck Veterinary Manual)

Smaller portions are more appropriate when the horse:

• Has a history of gastric ulcers

• Has insulin dysregulation

• Has previously developed laminitis

• Is changing onto a higher-starch product

• Has recurrent digestive problems

• Eats rapidly

• Is returning to feed after illness

A high-starch meal may reach its sensible limit well before the feed reaches 0.5% of body weight.

Why Are Sudden Grain Increases More Dangerous?

Starch is mainly digested in the small intestine.

When a horse receives more starch than can be effectively digested there, additional starch reaches the caecum and colon. Rapid fermentation can produce excess acid, disrupt normal fibre-digesting organisms and damage the hindgut environment.

A sufficiently large carbohydrate overload can contribute to:

• Colic

• Diarrhoea

• Hindgut acidosis

• Endotoxaemia

• Acute laminitis

Carbohydrate overload is a recognised cause of laminitis, but these severe consequences are most relevant to substantial grain exposure rather than every routine feed change. (Merck Veterinary Manual)

How Should Pasture Be Introduced?

Pasture is not simply “hay that is still growing.”

Fresh grass differs from conserved forage in moisture, sugar, protein, digestibility and fermentation characteristics. A horse that lived on pasture previously may still be unadapted to the pasture on your property.

A conservative introduction for a healthy, low-risk horse could begin with:

• Day 1: 15 minutes

• Day 2 or 3: 30 minutes

• Continue increasing in 15-minute steps only while the horse remains normal

Some extension protocols increase access daily, while more conservative plans hold each grazing duration for three to five days. The right speed depends on pasture richness and the individual horse. (OSU Extension Service)

Feed the horse some familiar hay before turnout so they are less likely to enter the paddock ravenous.

Monitor:

• Manure consistency

• Abdominal comfort

• Appetite

• Body weight

• Neck crest

• Hoof heat

• Digital pulses

• Willingness to walk and turn

Pasture Introduction for High-Risk Horses

Do not use a generic timetable for a horse with:

• Previous laminitis

• Insulin dysregulation

• Equine metabolic syndrome

• Obesity

• A large cresty neck

• Regional fat deposits

• PPID with abnormal insulin regulation

• Unexplained foot soreness

These horses may require:

• A dry lot

• Tested low-NSC hay

• A correctly fitted grazing muzzle

• Very limited or no pasture access

• Veterinary metabolic testing

• A structured weight-management plan

Pasture sugar and starch vary with plant species, season, growth stage, weather, drought and frost. Visual inspection cannot tell you whether the grass is safe for a metabolically vulnerable horse. (OSU Extension Service)

A grazing muzzle can reduce intake, but it does not make every pasture safe.

What If You Are Moving a Horse to a New Property?

Try to obtain at least 7 to 14 days of the horse’s previous hay and concentrate.

Ask for:

• Exact feed brand

• Product name

• Weight per meal

• Number of daily meals

• Hay type

• Approximate hay weight

• Pasture access

• Supplements

• Medication hidden in feed

• Previous colic or laminitis

• Dental problems

• Water source

Keep the first several days deliberately boring.

Provide familiar hay, clean water and the usual measured concentrate. Avoid immediately adding lush pasture, new supplements, large treats and a conditioning feed because the horse “looks a bit light.”

The move itself may already involve transport stress, reduced drinking, altered exercise and different housing. Preserving diet continuity removes at least one avoidable problem.

Does Water Matter During a Feed Change?

Yes.

Dry hay and concentrate require more drinking water than lush pasture. A horse moving from grass to dry forage may therefore need to obtain considerably more water directly from a bucket or trough.

Monitor actual intake, especially when:

• The water source tastes different

• The horse has moved yards

• Automatic waterers are unfamiliar

• Weather is cold

• The horse has travelled

• Group competition limits access

• More dry forage is being introduced

Poor water intake can reduce manure moisture and contribute to impaction over time. (University of Minnesota Extension)

A full trough proves that water exists.

It does not prove that the horse has been drinking it.

Should You Use a Probiotic During a Transition?

A probiotic should not be used as permission to change the diet faster.

Equine probiotic products vary widely in organisms, dose, stability and evidence. Some may be useful in particular situations, but no supplement can reliably cancel out:

• A sudden hay change

• A large grain increase

• Lush pasture exposure

• Dehydration

• Dental disease

• Feed contamination

• An intestinal obstruction

The foundation remains gradual change, adequate forage, clean water and appropriate meal size.

Special Cases That Need a Different Plan

Horses With Insulin Dysregulation or Previous Laminitis

These horses need both a gradual transition and strict control of sugar, starch and calories.

Do not introduce pasture or a new concentrate simply because the change is slow. A slowly introduced feed can still be unsuitable.

Use:

• Tested low-NSC forage

• Measured portions

• A low-calorie ration balancer where needed

• Controlled pasture access

• Close hoof monitoring

Hot feet, stronger digital pulses, reluctance to turn or short, stiff steps should be treated seriously. (OSU Extension Service)

Senior Horses and Horses With Poor Teeth

A horse that cannot chew the old ration properly may need a faster change for safety, but the new feed form should still be introduced in small quantities where possible.

Options may include:

• Soaked complete senior feed

• Soaked forage pellets

• Soaked hay cubes

• Beet pulp

• Chopped forage

Arrange a dental examination when the horse:

• Quids feed

• Drops partially chewed forage

• Eats slowly

• Produces long forage fibres in manure

• Loses weight

• Coughs during meals

• Has previously choked

Severely Underweight or Starved Horses

Do not use the standard transition chart for a severely emaciated horse.

Refeeding syndrome can produce dangerous metabolic and electrolyte changes. Rehabilitation begins with carefully weighed, frequent small meals under veterinary supervision, not free access to grain or a rich conditioning feed.

Research-based refeeding protocols commonly begin with small, frequent portions of good-quality alfalfa and increase intake gradually over the first 10 days. (Center for Equine Health)

Horses Recovering From Colic or Surgery

Follow the treating veterinarian’s instructions.

The correct progression depends on:

• The cause of the colic

• Whether surgery was performed

• Which intestine was affected

• Whether motility has returned

• Whether diarrhoea or reflux is present

• Current medication

• Manure production

Do not restart the usual concentrate because the horse appears hungry.

Foals and Growing Horses

Growing horses need consistent energy, quality protein and balanced minerals. Abrupt changes can disturb the digestive system, while poorly designed attempts to reduce calories can also leave the youngster deficient.

Changes should be planned around:

• Growth rate

• Body condition

• Forage analysis

• Calcium and phosphorus

• Copper and zinc

• Protein and lysine

• Exercise and soundness

How Worried Should You Be?

Low Risk

The horse:

• Is bright and eating

• Drinks normally

• Produces normal manure

• Has no colic or laminitis history

• Is changing feeds gradually

• Has no foot soreness

What to do: continue the transition and monitor the horse at least twice daily.

Moderate Risk

The horse has:

• Slightly softer manure

• Mildly reduced water intake

• A little more gas

• Slower eating

• Mild feed refusal

• A recent unavoidable hay change

What to do: pause the transition. Remain at the last tolerated ratio for 24 to 48 hours. Check temperature, appetite, drinking and manure.

Contact your veterinarian if the signs persist, recur or worsen.

High Risk

The horse develops:

• Reduced appetite

• Repeated loose manure

• Fewer or smaller manure piles

• Intermittent flank watching

• Repeated pawing

• Mild colic signs

• Increased hoof heat

• Stronger digital pulses

• Short or stiff steps

• A sudden large pasture or grain exposure

What to do: stop further dietary changes and contact your veterinarian the same day.

Critical

The horse has:

• Repeated or forceful rolling

• Persistent abdominal pain

• Marked sweating

• Severe abdominal distension

• Profuse watery diarrhoea

• Feed or saliva coming from the nostrils

• Inability to swallow

• Severe hoof pain

• Weakness or collapse

• Rapid deterioration

What to do: seek emergency veterinary care immediately.

When Is This an Emergency?

Suspected Colic

Call your veterinarian promptly if the horse shows:

• Repeated flank watching

• Pawing

• Lying down repeatedly

• Rolling

• Sweating without exercise

• Refusal to eat

• Little or no manure

• Abdominal distension

• Pain that returns after medication

• Weakness or collapse

Some horses needing emergency surgery initially show only mild or moderate pain. Visible drama does not reliably identify the seriousness of the intestinal problem. (Merck Veterinary Manual)

Accidental Grain Overload

Treat access to a feed room, grain bin or concentrate bag as an emergency even if the horse still appears normal.

Immediately:

1. Remove access.

2. Call your veterinarian.

3. Estimate what and how much was eaten.

4. Record when the exposure occurred.

5. Save the feed label and batch details.

6. Keep the horse quiet.

7. Do not exercise the horse.

8. Do not give mineral oil or medication unless directed.

9. Monitor manure, digital pulses and hoof temperature.

Do not wait for colic or laminitis to become obvious.

Possible Laminitis

Emergency warning signs include:

• Hot feet

• Bounding digital pulses

• Short, pottery steps

• Reluctance to turn

• Weight shifting

• Standing rocked back

• Refusal to walk

Acute laminitis can progress to separation of the hoof laminae and displacement of the coffin bone. Early treatment matters. (Merck Veterinary Manual)

Choke

Feed or saliva draining from the nostrils, repeated coughing, gulping, neck stretching or inability to swallow suggests oesophageal obstruction.

Remove both food and water and call your veterinarian. Do not pour oil, water or medication into the horse’s mouth because aspiration can occur. (Merck Veterinary Manual)

Severe Diarrhoea

Profuse watery diarrhoea, especially with fever, depression, colic or weakness, requires urgent veterinary assessment.

Isolate the horse from other horses until an infectious cause has been considered.

What Should You Do if You Changed the Feed Too Quickly?

If the horse is currently normal:

1. Stop increasing the new feed.

2. Return to the last tolerated proportion.

3. Keep clean water continuously available.

4. Avoid pasture, treats and additional supplements.

5. Continue familiar forage.

6. Monitor appetite and manure twice daily.

7. Check the feet and digital pulses.

8. Restart the transition more slowly after several stable days.

If the horse has mild digestive changes:

• Contact your veterinarian for advice.

• Check rectal temperature.

• Record water intake.

• Photograph the manure.

• Note the exact old and new feeds.

• Keep labels and ingredient lists.

If the horse has pain, marked appetite loss, choke signs, diarrhoea or foot soreness, stop managing it as a routine feed transition and arrange veterinary assessment.

Common Mistakes Horse Owners Make

Changing to a “Better” Feed Overnight

A more expensive or nutritionally appropriate product can still cause trouble when introduced abruptly.

Forgetting That Hay Counts

The hindgut notices a change in hay even when the new bale looks almost identical.

Introducing Pasture Too Quickly

Going from dry hay to several hours of lush pasture is a major dietary change.

Feeding by Scoop

Feed density varies. Weigh the ration.

Increasing Grain to Put Weight On Quickly

Investigate why the horse is thin before adding large quantities of concentrate.

Changing Several Things Together

Hay, concentrate, supplements and pasture should not all change simultaneously unless there is an urgent medical reason.

Assuming a Probiotic Makes the Change Safe

No powder can reliably neutralise an unsuitable or abrupt transition.

Ignoring Early Hoof Signs

A mildly stiff turn may be the first warning of laminitis, not simple laziness.

Waiting for Dramatic Colic

Reduced appetite, dry manure and mild flank watching can precede more obvious pain.

Running Completely Out of the Old Feed

Keep a reserve large enough to complete the transition whenever possible.

How Can Feed-Change Problems Be Prevented?

• Keep written feeding records.

• Weigh hay and concentrate.

• Photograph feed labels.

• Keep a reserve of the current hay and feed.

• Change one major ingredient at a time.

• Allow at least 7 to 14 days.

• Extend the transition for vulnerable horses.

• Introduce pasture in minutes rather than hours.

• Maintain unrestricted clean water.

• Feed concentrate in small meals.

• Avoid sudden starch increases.

• Monitor appetite and manure daily.

• Check body condition regularly.

• Check digital pulses during pasture introduction.

• Maintain dental care.

• Secure the feed room.

• Investigate unexplained weight loss before adding calories.

AAEP colic-prevention guidance similarly prioritises a consistent routine, a forage-based diet, smaller concentrate meals, water access and appropriate turnout. (AAEP)

A Simple Feed Transition Record

Track:

Date	Old feed amount	New feed amount	Hay	Pasture time	Water intake	Manure	Hoof comfort
Day 1	75%	25%	Normal	None	Normal	Normal	Normal
Day 3	75%	25%	Normal	15 min	Normal	Normal	Normal
Day 4	50%	50%	Normal	30 min	Normal	Slightly soft	Normal

This prevents several people from making separate adjustments because each assumes someone else forgot to increase the feed.

Stable communication: protecting horses from the committee-based ration.

Will My Horse Be Okay?

Most healthy horses tolerate gradual diet changes without difficulty.

The outlook is best when:

• The previous diet is known.

• The change is made slowly.

• Forage remains the foundation.

• Water intake stays normal.

• Meal sizes remain small.

• Pasture access is controlled.

• Early abnormalities are taken seriously.

Mild manure softening may settle when the transition is paused. Persistent appetite loss, pain, profuse diarrhoea, choke signs or hoof soreness requires veterinary assessment rather than another ration adjustment.

FAQs About Changing a Horse’s Diet

How quickly can I change my horse’s feed?

Allow at least 7 to 14 days for most hay or concentrate changes. Replace approximately 20% to 25% every two to three days. Horses with digestive or metabolic problems may need a longer transition. (University of Minnesota Extension)

Does changing to a new batch of the same hay require a transition?

Yes. Different batches can vary in species, maturity, fibre, sugar, protein and energy. Mix the old and new hay gradually whenever possible.

How quickly can I introduce my horse to pasture?

Begin with approximately 15 minutes and increase in small steps. Healthy horses may progress more quickly than ponies, obese horses or horses with previous laminitis, which may need a much stricter veterinary plan. (OSU Extension Service)

Is soft manure normal during a feed change?

A small temporary change can occur, but persistent or watery diarrhoea is not something to dismiss. Pause the transition and contact your veterinarian if it continues, worsens or is accompanied by fever, pain or reduced appetite.

Should I give a probiotic while changing feed?

It may be used when your veterinarian or nutritionist recommends a specific product, but it does not replace a gradual transition. The rate and suitability of the diet change still matter most.

Final Thoughts

Changing a horse’s diet safely is less about finding the perfect feed and more about giving the digestive system enough time to adjust to it.

The key rules are:

• Treat every new hay batch as a feed change.

• Replace only 20% to 25% at a time.

• Allow 7 to 14 days, or longer for vulnerable horses.

• Change one major component at a time.

• Introduce pasture cautiously.

• Weigh concentrate rather than feeding by scoop.

• Monitor water, manure and hoof comfort.

• Treat grain overload, choke, colic and laminitis signs as emergencies.

The four-day microbiome study does not mean a four-day transition is safe for every horse. It means the hindgut begins reacting almost immediately.

Slow and steady is not exciting.

Neither is an emergency colic surgery invoice, which is rather the point.

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If you are changing hay, pasture or concentrate and are unsure how quickly to proceed, ASK A VET™ can help you organise the existing ration, transition schedule and warning signs while your local veterinarian provides hands-on assessment for colic, diarrhoea, choke or laminitis.