Horse Diet and Behavior: How Starch and Feeding Choices Affect Reactivity

By Dr Duncan Houston

If your horse feels “hot,” reactive, or difficult to settle, diet is one of the first things that should be reviewed.

Many behavioural issues in horses are not purely training problems. They are often influenced by how energy is delivered through the diet.

In practice, one of the most common patterns is this:
high-starch diets increase reactivity, while forage-based, low-starch diets produce more consistent behaviour.

Quick Answer

High-starch diets can increase excitability and reactivity in horses by causing rapid glucose and insulin spikes. Lower-starch, higher-fiber or fat-based diets tend to provide more stable energy and are often associated with calmer, more manageable behaviour.

Why Diet Affects Behaviour More Than People Realise

Horses are designed to process slow, steady energy from forage.

When this is replaced with:

• high-starch grain

• large concentrate meals

the metabolic response changes.

Instead of steady energy, the horse experiences:

• rapid glucose spikes

• increased insulin response

• fluctuating energy levels

This can translate into:

• increased reactivity

• reduced focus

• inconsistent behaviour

In practice, this is often mislabelled as a training issue rather than a feeding issue.

What High-Starch Diets Actually Do

Starch is digested quickly in the small intestine.

This leads to:

• rapid energy availability

• hormonal changes

• potential gut disruption if excess starch reaches the hindgut

The behavioural impact can include:

• heightened alertness

• quicker reactions

• reduced tolerance to pressure

Not every horse responds the same way, but sensitive horses show this very clearly.

What Lower-Starch Diets Do Differently

When energy is supplied through:

• forage

• fibre

• fat

energy release is slower and more consistent.

This supports:

• stable metabolism

• more predictable behaviour

• improved focus under saddle

This does not make a horse “lazy.”
It makes energy more controlled and usable.

How Serious Is Diet-Related Reactivity?

Low Impact

• slightly forward or energetic

• manageable under normal conditions

What this means: diet may be contributing but not dominant

Moderate

• inconsistent behaviour

• difficulty relaxing

• overreaction to aids

What this means: diet is likely influencing behaviour

High Impact

• strong reactivity

• poor rideability

• difficulty settling or focusing

What this means: diet is a major contributing factor

Complex Cases

• behavioural issues persist despite diet changes

What this means: training, pain, or environment also need investigation

Common High-Starch Feeds Linked to Reactivity

Higher-risk feeds include:

• oats

• sweet feeds

• corn-based mixes

• high-starch pellets

These are not inherently harmful, but they are often overused or poorly matched to the horse’s needs.

Lower-Starch Feeding Options

Better options for many horses include:

• hay-based diets

• beet pulp without molasses

• fibre-based feeds

• fat supplementation such as oils or flax

These provide energy without sharp metabolic spikes.

Feeding Strategies That Improve Behaviour

Prioritise Forage

This should always be the foundation.

• supports gut health

• stabilises energy release

• reduces behavioural fluctuation

Reduce Starch Intake

Lowering grain levels often produces noticeable behavioural changes within weeks.

Use Fat as an Energy Source

Fat provides:

• slow-release energy

• no rapid glucose spikes

This is particularly useful for performance horses needing calories without reactivity.

Keep Feeding Consistent

Sudden dietary changes can:

• disrupt gut function

• affect behaviour

Consistency is critical.

Support Gut Health

The gut and brain are closely linked.

Maintaining a stable gut environment supports:

• comfort

• behavioural stability

What To Do Right Now

If your horse is reactive or difficult to manage:

• review how much grain is being fed

• assess total starch intake

• increase forage proportion

• consider shifting toward fibre and fat-based energy sources

• monitor changes over 2 to 3 weeks

If behaviour improves with dietary change, that confirms diet was a key factor.

When Is This Not Just a Diet Issue?

Diet is important, but not everything.

You should investigate further if:

• behaviour is extreme or worsening

• there are signs of pain or discomfort

• performance drops suddenly

• no improvement occurs after diet adjustment

Behaviour is often multi-factorial.

Common Mistakes

• assuming all behaviour is training-related

• feeding high-energy diets to horses that do not need them

• making sudden diet changes

• overlooking forage quality

• expecting instant results

In practice, many “hot” horses are simply overfed starch.

Long-Term Management

The goal is not to remove energy, but to deliver it correctly.

Focus on:

• forage-based nutrition

• appropriate calorie levels

• controlled starch intake

• consistent feeding routine

When this is done well, behaviour becomes more predictable and manageable.

Frequently Asked Questions

Will changing diet fix all behaviour problems?
No, but it can significantly reduce reactivity linked to feeding.

How quickly will I see a difference?
Often within 1 to 3 weeks as metabolism stabilises.

Do performance horses need grain?
Some do, but many can perform well on lower-starch diets with alternative energy sources.

Can diet make a horse spooky?
It can contribute by increasing reactivity and sensitivity.

Is fat feeding safe?
Yes when balanced correctly. It is a useful alternative energy source.

Final Thoughts

Diet has a direct effect on how a horse feels and responds.

The key questions are:

• how quickly is energy being delivered

• how stable is the metabolic response

• is the diet matched to the horse’s actual workload

When feeding aligns with how the horse’s body is designed to function, behaviour often improves without needing to change everything else.

If you are unsure whether your horse’s diet is contributing to behavioural issues or want help adjusting feeding for better focus and rideability, ASK A VET™ can guide you through a practical, tailored plan.