Diet Management for PSSM

PSSM—Polysaccharide Storage Myopathy—is a condition where a horse's muscles store too many carbohydrates and don't process them efficiently. When the muscle tries to use all those carbs during work, something goes wrong. The muscle fatigues, locks up, and ties up. It hurts. The horse can't move properly, sometimes sweats excessively, might run a fever, and is miserable.

The muscle damage from a tie-up episode can be severe. Myoglobin released from damaged muscle goes into the urine, which is visible as dark urine. Severe episodes cause kidney damage and potentially death. Even mild episodes cause muscle soreness and pain. This is serious. If you have a horse that ties up, managing his diet is the first-line treatment.

PSSM horses have a metabolic problem—their muscles don't handle carbohydrates the way normal horse muscles do. Feeding them a normal high-carb diet sets them up for tie-ups. The solution is to feed them a low-carbohydrate diet that doesn't accumulate in the muscle and trigger problems.

The interesting thing is that a PSSM horse on the right diet can work well and not tie up. A PSSM horse on the wrong diet—high carbs—will tie up regularly, sometimes severely. Diet management is genuinely the difference between a horse that can perform and a horse that's unsafe to ride.

Not every horse that ties up has PSSM. Some tie-ups are from electrolyte imbalances, dehydration, or exertional rhabdomyolysis. But if a horse is tying up regularly during or after work, especially if it's worse when he's having a lot of grain or days off, PSSM is the likely culprit. Talk to your vet about getting diagnosed.

This guide explains how PSSM horses are fed, what feeds are safe, and what mistakes to avoid so your horse can work without tying up.

PSSM horses have muscles that accumulate polysaccharides (complex carbohydrates) at high levels. When they work, those polysaccharides are supposed to be broken down for energy. But in PSSM horses, the breakdown process isn't efficient, so metabolic byproducts accumulate and damage the muscle. The result is the muscle fatigue, pain, and tying up that owners see.

The metabolic problem varies in severity. Some PSSM horses have severe disease and tie up on minimal work; others have mild disease and only tie up under stress. Genetic testing and muscle biopsies can diagnose PSSM and determine severity, which guides how strict the diet needs to be.

The treatment is to reduce the carbohydrates the muscle encounters so there's less polysaccharide accumulation in the first place. The lower the carb load, the fewer polysaccharides build up, and the less likely a tie-up episode. This is why PSSM horses do well on low-carbohydrate diets.

Carbohydrates come in different forms. Simple sugars (like molasses) are fast carbs that spike blood glucose quickly. Complex carbohydrates (like starch in grain) are slower but still present in high amounts in typical horse feed. Both can be problematic for PSSM horses, but simple sugars are generally worse. Fiber—the carbs in forage—has a different metabolic pathway and is usually tolerated better.

For a PSSM horse, the ideal diet is high in fat (which provides energy without being carbohydrate-based) and low in NSC—non-structural carbohydrates. NSC includes sugars and starch. A PSSM horse needs an NSC of less than 10 percent ideally, with some cases needing less than 5 percent. Normal horse feed is often 20 to 30 percent NSC, which is too much.

Fat is crucial for PSSM horses because it provides energy without carbohydrate overload. Adding oil to the diet increases calories without increasing carbs. This lets a PSSM horse maintain weight and energy without carbohydrate problems. Oils, rice bran, and other fat sources work well.

Fiber—the long-stem forage—is generally safe for PSSM horses. Pasture and hay have carbs in them, but mostly in the form of fiber that doesn't accumulate in muscle the way grain carbs do. A PSSM horse can eat good hay and grazing grass without the same concerns as eating grain.

Electrolytes become more important for PSSM horses because muscle damage from tie-ups causes electrolyte losses, and sweating from the stress of episodes depletes electrolytes. Maintaining good electrolyte balance helps prevent episodes and supports recovery.

Exercise actually helps PSSM horses. Regular work prevents polysaccharide accumulation better than rest. A PSSM horse should be worked regularly—daily if possible—rather than having days off. This is opposite to many horses and is specific to the PSSM metabolism.

The foundation of a PSSM diet is good-quality hay. Grass hay or mixed hay is fine; alfalfa is fine. Test your hay if possible to know the NSC level. Most grass hays are under 15 percent NSC and work well for PSSM horses. Some hays are higher, so testing is worthwhile if you want to be sure.

Remove grain and replace it with fat sources. Instead of five pounds of grain daily, give zero grain plus oil or another fat source. Start with a quarter cup of oil daily and gradually increase to a half cup or more, depending on the horse's size and needs. The oil provides calories the horse needs for work without the carbohydrate load.

Rice bran is another option. It's high in fat and lower in NSC than grain. Some commercial products are specifically formulated for PSSM horses with fat as the base and NSC under 10 percent. Ask your vet or grain store for recommendations on commercial PSSM feeds.

Avoid products with molasses, corn, sweet grains, or other high-sugar additions. Read the label. If you see molasses as one of the first ingredients, it's not PSSM-safe. If you see corn or barley, it's probably too high in NSC. Plain oats are lower in NSC than some grains but still might be too high for severe PSSM; talk to your vet about whether oats are safe for your horse.

Supplement with a vitamin and mineral balancer because you're removing grain, which usually provides those nutrients. A PSSM-specific mineral supplement formulated to work with low-NSC diets ensures your horse isn't deficient.

Add salt and electrolytes, especially if your horse is working regularly or in hot weather. Electrolyte losses from PSSM-related sweating are real, and replacing them prevents cramping and supports muscle function. Mix electrolytes in food or water.

Water access is critical. A PSSM horse needs unlimited water at all times. Dehydration worsens metabolic problems and increases tie-up risk. If water freezes in winter, break ice multiple times daily or use water heaters.

Exercise regularly. A PSSM horse should work four to seven days a week, depending on severity and fitness. Regular work clears polysaccharides from muscle and prevents the accumulation that causes tie-ups. A PSSM horse rested for days off is at higher risk for tying up when work resumes because the polysaccharides have accumulated.

Make any feed changes gradually over two to three weeks. PSSM horses seem sensitive to digestive changes, so transition slowly from the old diet to the new low-NSC diet.

If your horse ties up despite these changes, call your vet. The diet might not be low-NSC enough, the horse might have a different problem altogether, or there might be other management issues causing episodes.

Performance horses with PSSM need to manage their diet while still maintaining the fitness and energy for work. Low-NSC feeding works, but finding a commercial product that meets both the low-carb requirement and the performance horse energy need takes some work. Talk to your vet about products formulated for performance PSSM horses.

Idle or light-work PSSM horses sometimes do well on just hay and minimal supplementation because they're not working hard enough to accumulate significant polysaccharides. But regular light work is usually better than complete rest for a PSSM horse, even if the horse isn't being ridden hard.

Breeding mares with PSSM need careful nutrition management because their energy needs increase during late pregnancy and lactation. They need fat-based feeds formulated for mares that are also low-NSC. A mare nursing a foal needs good nutrition and energy, but carb sources must be carefully chosen.

Young horses that develop PSSM symptoms need diet management but also must grow and develop. A growing PSSM horse needs adequate protein and minerals for growth plus low-NSC carbs. This sometimes requires feeding multiple supplements to meet all the needs. Consult your vet about balancing growth nutrition with PSSM management.

Senior PSSM horses sometimes have weaker digestion and might not tolerate high-fat diets as well as younger horses. Feeding senior PSSM horses might require products specifically formulated for senior PSSM horses with digestibility in mind. Poor-quality hay becomes more problematic for older horses, so good forage quality matters more.

Hard keeper PSSM horses need high calories without high carbs. Fat becomes essential because oil provides calorie density without carbohydrates. These horses need even more oil or fat sources than easy keeper PSSM horses just to maintain weight. Combined with exercise requirements, feeding a hard keeper PSSM horse takes work.

Easy keeper PSSM horses sometimes have the opposite problem—they maintain weight even on minimal calories, which can make it hard to add enough fat for energy without overfeeding calories. These horses might need less total amount of feed despite high-fat content.

The biggest mistake is trying to feed a PSSM horse a normal diet and expecting him not to tie up. It won't work. The horse will tie up, sometimes badly, and that's not fair to him. Accept that PSSM requires a different diet and commit to feeding it properly.

Feeding grain to a PSSM horse when it's not NSC-tested is dangerous. You might think the grain is low-NSC, but it might not be. Some grains marketed as "low-starch" are still too high in NSC for PSSM horses. Testing your grain's NSC or feeding only products you know are safe is essential.

Not enough exercise worsens PSSM problems. A PSSM horse that's rested for several days off develops polysaccharide accumulation, and the next work session brings on a tie-up. Regular work, even light work, prevents this. Make regular exercise non-negotiable for PSSM horses.

Diet management without exercise rarely fully controls PSSM. The combination of low-NSC diet plus regular work is what really prevents tie-ups. You need both, not one or the other.

Sudden feed changes can trigger tie-ups even in horses on the right diet. Never make a sudden switch from one feed to another. Transition over three weeks minimum, adding the new feed while reducing the old.

Ties-ups during competition or high stress despite a low-NSC diet might indicate that other factors are involved. Dehydration, electrolyte imbalance, or muscle fatigue from intense work can cause tie-ups even in managed PSSM horses. Ensuring adequate water, electrolytes, and appropriate fitness helps prevent these.

Assuming that all PSSM horses need the same diet doesn't work. Severity varies. Some horses can tolerate small amounts of grain; others can't tolerate any. Some can manage on hay plus oil alone; others need commercial feeds. Your vet can help determine how strict your specific horse needs to be.

Ignoring warning signs of inadequate nutrition while managing PSSM creates problems. A PSSM horse on too-restrictive a diet might become thin and weak. If your horse is losing weight or energy despite the PSSM diet, the diet is too restrictive or incomplete for his needs. Work with your vet to adjust it.

Get your horse tested for PSSM if he's tying up regularly. A muscle biopsy or genetic test confirms diagnosis and identifies severity. Treating based on diagnosis is much better than guessing.

Find a vet who understands PSSM and can guide your feeding decisions. Not all vets are equally knowledgeable about PSSM management. A vet experienced with PSSM can recommend specific products and help troubleshoot if problems arise.

Have your hay tested for NSC if possible. This costs $20-40 and tells you whether your current hay is safe or if you need to find different hay. Knowing what you're feeding removes guesswork.

Keep a detailed log of what you're feeding and how your horse responds. Note his energy, whether he ties up, his condition, and any other relevant information. This helps you identify patterns and know whether changes you make are helping.

If your horse ties up despite diet management, write down the circumstances—what he ate, how much exercise, stress level, temperature. Patterns emerge that might point to what's triggering tie-ups.

Be consistent with exercise. A PSSM horse does much better on a consistent schedule than with sporadic work. If you usually ride four days a week, stick to that. If you can only ride three days a week, do three consistently rather than sometimes two and sometimes five.

Manage electrolytes especially carefully during hot weather or competition. Sweating in heat depletes electrolytes more, and PSSM horses might be prone to more sweating. Adding extra electrolytes during these times helps prevent episodes.

Join a PSSM owner group or online community. Other owners managing the condition can share what works, what products are good, and how to troubleshoot problems. Learning from others' experience is valuable.

Expect management to take commitment. A PSSM horse isn't a low-maintenance horse. He needs consistent exercise, careful feeding, and regular monitoring. If you're willing to do that, he can have a good quality of life. If you're not, reconsider whether managing this horse is realistic.