Tying-Up in Horses: The Nutritional Connection

What Is Tying-Up in Horses?

Tying-up — also known as exertional rhabdomyolysis, azoturia, or Monday morning disease — is a painful muscle condition that affects horses during or shortly after exercise. The horse's large muscle groups, particularly those over the hindquarters and back, cramp severely and may become rock-hard to the touch.

Symptoms of tying-up include:

• Reluctance or refusal to move
• A short, stiff gait, especially in the hindquarters
• Excessive sweating out of proportion to workload
• Rapid breathing and elevated heart rate
• Dark or discoloured urine (a sign of myoglobin release from damaged muscle cells)
• Visible muscle tremors or fasciculations

Tying-up can range from mild stiffness to a life-threatening emergency. In severe cases, massive muscle breakdown releases myoglobin into the bloodstream, which can damage the kidneys. It's a condition every horse owner should take seriously — and one where nutrition plays a far bigger role than many people realise.

Why Does Tying-Up Happen?

To understand the nutritional connection, it helps to understand the basic muscle physiology involved. Muscles need energy to contract and, critically, to relax. That energy comes from glycogen (stored glucose) and fat. When something goes wrong with how a horse stores, uses, or recovers from burning that energy, the result can be painful muscle cramping and cell damage.

There are two broad categories of tying-up:

Sporadic Exertional Rhabdomyolysis

This type occurs as isolated episodes and is typically triggered by identifiable factors such as:

• Overexertion relative to fitness level
• A sudden return to hard work after rest (the classic "Monday morning" scenario)
• A diet too high in starch or sugar for the workload
• Electrolyte imbalances
• Stress, excitement, or viral illness

Chronic Exertional Rhabdomyolysis

Some horses have an underlying genetic predisposition that makes them repeatedly susceptible. The two most well-known forms are:

• Polysaccharide Storage Myopathy (PSSM) — a condition where muscles store excessive or abnormal glycogen. Common in Quarter Horses, Draught breeds, Warmbloods, and their crosses.
• Recurrent Exertional Rhabdomyolysis (RER) — most commonly seen in Thoroughbreds, Standardbreds, and Arabians, often linked to excitable temperaments and triggered by stress or high-grain diets.

Regardless of the type, nutrition is one of the most powerful tools we have for both prevention and management.

The Nutritional Triggers Behind Tying-Up

Excess Starch and Sugar

This is the single biggest dietary risk factor for tying-up episodes. When a horse eats a high-starch meal — such as a large feed of cereal grains like oats, barley, or commercial mixes heavy in cereals — blood glucose and insulin spike. This drives glycogen storage in the muscles.

For a horse with PSSM, this is particularly dangerous because their muscles already store too much glycogen. But even horses without a genetic condition can tie up if they're fed a high-energy cereal-based diet and then worked inconsistently.

The classic scenario: a horse is fed a full ration of hard feed on a rest day, then ridden hard the next morning. The muscles are loaded with glycogen, and the stage is set for an episode.

Key dietary guideline: For horses prone to tying-up, total non-structural carbohydrate (NSC) intake — which includes starch and water-soluble sugars — should ideally be kept below 10-12% of the total diet on a dry matter basis.

Insufficient Fat in the Diet

If you reduce starch, you need to replace those calories with something — and fat is the answer. Fat provides slow-release energy without the insulin and glycogen spikes associated with cereals.

Research has shown that shifting a horse's energy source from starch to fat can dramatically reduce tying-up episodes, particularly in horses with PSSM and RER. The muscles adapt to burning more fat for fuel, which reduces their reliance on glycogen.

Good sources of supplemental fat for horses include:

• Vegetable oil (such as soya or canola oil) — start with small amounts and build up gradually
• Rice bran (stabilised)
• Linseed (flaxseed) — also provides omega-3 fatty acids
• Copra meal (coconut meal) — high in fat and fibre, low in starch

Aim for fat to provide at least 15-20% of the total calorie intake for horses with chronic tying-up conditions.

Electrolyte Imbalances

Electrolytes — sodium, chloride, potassium, calcium, and magnesium — are essential for normal muscle contraction and relaxation. An imbalance in any of these can increase the risk of tying-up.

• Sodium and chloride are lost heavily in sweat. Horses in regular work need daily salt supplementation (typically 1-2 tablespoons of plain table salt or free access to a salt lick).
• Potassium is usually adequate in forage-based diets but can become depleted in heavily sweating horses.
• Calcium and magnesium play direct roles in muscle function. Calcium triggers muscle contraction; magnesium helps muscles relax. A marginal deficiency in either can predispose to cramping.

Many horse owners underestimate how much salt their horse needs, particularly in warmer weather or during regular training. A horse in moderate work can lose 50-70 grams of salt per day through sweat alone.

Vitamin E and Selenium Deficiency

Vitamin E and selenium are powerful antioxidants that protect muscle cell membranes from oxidative damage during exercise. Deficiency in either nutrient has been linked to increased susceptibility to muscle problems, including tying-up.

• Vitamin E is found in fresh pasture but degrades rapidly in stored hay and hard feed. Horses on hay-based diets or limited turnout often need supplementation. Horses prone to tying-up may benefit from 2,000-5,000 IU of natural vitamin E (d-alpha-tocopherol) daily, depending on body weight and severity.
• Selenium levels vary enormously depending on soil type and geography. In many parts of the UK, Ireland, and certain regions of Australia and the US, soils are selenium-deficient, meaning hay and pasture grown on those soils will also be low. Blood testing is the most reliable way to assess selenium status.

Caution: Selenium is toxic in excess. Never supplement selenium without knowing your horse's current intake from forage, hard feed, and any other supplements. Over-supplementation can cause serious harm.

Inadequate Forage

Forage — hay, haylage, or pasture — should form the foundation of every horse's diet, typically at least 1.5-2% of body weight per day in dry matter. Forage provides slow-release energy from fibre fermentation in the hindgut, which avoids the insulin and glycogen spikes associated with starch.

For horses prone to tying-up, maximising forage intake and minimising concentrate feeds is a cornerstone of management. If your hay is high in sugar (which can happen with certain grass species or cutting conditions), soaking it for 30-60 minutes before feeding can reduce the water-soluble sugar content by 20-50%.

Building a Tying-Up Prevention Diet

Here's a practical framework for feeding a horse prone to tying-up:

Step 1: Maximise Forage

Feed ad-lib or near ad-lib hay or haylage. Choose mature, later-cut hay where possible, as it tends to be lower in sugar. If using haylage, opt for higher dry matter varieties.

Step 2: Eliminate or Drastically Reduce Cereal Grains

Remove oats, barley, maize, and cereal-based mixes from the diet. Replace with low-starch alternatives such as unmolassed sugar beet pulp, soya hulls, or commercial low-starch feeds specifically formulated for muscle-sensitive horses.

Step 3: Add Fat for Energy

If the horse needs additional calories for work or condition, add oil or high-fat feeds. Introduce oil slowly — start with 50ml per day and build up over 2-3 weeks to avoid digestive upset. Most horses in moderate work do well on 100-200ml of oil per day.

Step 4: Supplement Electrolytes Daily

Provide 1-2 tablespoons of plain salt daily in feed, and additional electrolytes during periods of heavy sweating. Don't rely solely on a salt lick — research shows many horses don't lick enough to meet their requirements.

Step 5: Ensure Adequate Vitamin E and Selenium

Supplement natural vitamin E, particularly if the horse has limited access to fresh pasture. Have selenium levels checked via a blood test before supplementing, and use an appropriate dose based on results.

Step 6: Check for Magnesium Adequacy

Magnesium deficiency is more common than many owners realise, particularly in horses on limited or low-quality pasture. Supplementing 5-10 grams of magnesium oxide daily is generally safe and can help support normal muscle function.

Step 7: Analyse the Whole Diet

The most important step is often the one most people skip: looking at the diet as a whole. Individual supplements and feed changes are only effective if they fit together into a balanced overall ration. Consider analysing your horse's diet to identify any gaps, excesses, or imbalances that could be contributing to muscle problems. A proper diet analysis takes the guesswork out of nutrition and gives you a clear picture of what your horse is actually getting versus what they need.

What to Do If Your Horse Ties Up

If your horse shows signs of tying-up during or after exercise:

1. Stop exercise immediately. Do not force the horse to walk it off — this can cause further muscle damage.
2. Keep the horse calm and warm. Throw a rug over the hindquarters if the weather is cold.
3. Call your vet. They can assess severity, provide pain relief, and take blood samples to measure muscle enzyme levels (CK and AST).
4. Offer water but do not feed until advised by your vet.
5. Do not transport the horse unless your vet advises it — the stress and muscle effort of balancing in a trailer can worsen the episode.

After an episode, work with your vet to determine the underlying cause. If episodes are recurrent, genetic testing for PSSM (via a hair or blood sample) is strongly recommended.

The Bigger Picture: Exercise and Management

While this article focuses on nutrition, it's important to acknowledge that diet doesn't exist in isolation. Successful management of tying-up also requires:

• Consistent exercise — avoid the "feast and famine" pattern of hard work followed by days off with full feed.
• Adequate turnout — horses that stand in stables for long periods are at higher risk. Daily turnout, even if limited, helps.
• Gradual fitness building — avoid sudden increases in workload intensity.
• Stress management — particularly for RER-prone horses, minimising environmental and social stress can reduce episodes.

Final Thoughts

Tying-up is a complex condition, but the nutritional connection is clear and well-supported by research. For many horses, simply shifting from a high-starch diet to a forage-and-fat-based diet can be transformative — reducing or even eliminating episodes entirely.

The key principles are straightforward: feed plenty of forage, avoid cereal grains, add fat for energy, keep electrolytes topped up, and ensure adequate antioxidant status with vitamin E and selenium. If your horse has been diagnosed with PSSM or RER, these dietary changes aren't optional — they're the foundation of effective management.

Every horse is an individual, and the best dietary approach depends on breed, workload, temperament, and any underlying conditions. When in doubt, work with your vet and consider a professional diet analysis to make sure you're covering all the bases. Your horse's muscles will thank you.