Feeding Advice for Horses with Tying Up & PSSM

Previously known as Tying-Up, Azoturia and Monday Morning Disease, Equine Rhabdomyolysis Syndrome or ERS is the most common name currently used to describe muscle disorders in the horse. Studies and research have identified some distinct disease pathways in horses and we will explain more about what those are. But first….

How big a problem is tying up in horses?

Surveys in leisure horses suggest up to 3% of the population may be affected by tying up and other exercise-related muscle disorders, whilst in performance horses the incidence rises to between 5 and 7% of racehorses, 8% in polo ponies and up to 14% in eventers. The differences between studies of different disciplines probably reflect variability in study designs etc and it is probably fair to suggest that an incidence of around 10% in exercising horses is a reasonably accurate assessment based on research to date.

Why does Equine Rhabdomyolsis syndrome happen?

Horses affected have an underlying susceptibility that is triggered by other things. It is generally accepted that movement or exercise is the final trigger factor that results in the muscle seizing or cramping but there are other risk factors also involved including diet, sudden changes to work without adjusting the diet, electrolyte imbalances, infections and weather. Several of these may combine to create a “perfect storm”.

How are muscle problems in horses defined?

Some horses have an inherent muscle defect which can either be a disorder relating to the contracting and relaxing of muscles which tends to be called RER, or a defect in how carbohydrates are stored and/or utilised in the muscles which is referred to as PSSM. The latter is further sub-divided into two groups – Type 1 and Type 2.

There appears to be a group of sufferers that don’t have an underlying muscle defect. Research suggests that inappropriate diets and management issues are most likely to be the cause of problems in this group.

More about the different disease types

Polysaccharide Storage Myopathy (PSSM)

PSSM in horses can be divided into two sub-groups:

PSSM1– caused by a genetic mutation found in more than 20 breeds of horses with the highest prevalence being in European draft horses. Quarter horses and Appaloosas are also affected by PSSM1. These horses don’t break down glycogen in their muscles as easily as normal horses due to a mutation of the enzyme Glycogen Synthase 1 (GYS1). This means they can have 1.5 – 5 times higher levels of muscle glycogen in their muscles compared to normal horses. This can make it very difficult and painful for them to move.

Whilst some PSSM1 horses are asymptomatic, diets high in non-structural carbohydrates (sugar and starch) can exacerbate clinical signs as these nutrients are stored and accumulate. Signs include apparent laziness, shifting lameness, tensed up abdomen, tremors in flank area and firm hard muscles over hindquarters. More chronic signs such as reluctance to move forward, muscle loss and lack of energy are seen in Draft Breeds and gait abnormalities can also be seen in Quarter Horses.

PSSM2 – refers to all PSSM cases that aren’t caused by genetic mutation GYS1 and so there may be more than one further subset identified in due course. PSSM2 is more prevalent in Warmbloods and is often detected via poor performance.

PSSM2 – refers to all PSSM cases that aren’t caused by genetic mutation GYS1 and so there may be more than one further subset identified in due course. PSSM2 is more prevalent in Warmbloods and is often detected via poor performance. It is interesting to note that leading researchers such as Stephanie Valberg and Richard Piercy who presented at conferences in 2022 (European Workshop on Equine Nutrition and BEVA Congress respectively) both stated that there is no genetic test for PSSM2 cases.

The most common signs of PSSM2 in horses are poor performance and a drop in energy levels after a short period of exercise. Unwillingness to perform can be highlighted by the horse’s reluctance to collect and engage hindquarters, as well as poor rounding over fences. Firm back and hindquarter muscles can also be seen in some cases.

Recurrent Exertional Rhabdomyolysis (RER)

RER is the term used to describe horses that have an abnormal process of muscle contraction. As the name suggests it commonly happens when horses are working at speed and so is most commonly seen in Thoroughbreds, Standardbreds and Arabs. It also tends to be associated with nervous, excitable or stressy individuals and in younger horses, fillies are more prone to the problem but this difference between the sexes seems to reduce with age. The trigger factors for RER include high cereal diets and being held back when training at speed.

If we use ERS to refer to those horses that don’t have a muscle abnormality, then the most commonly reported trigger factors include:

  • High starch diets and often too little fibre
  • Electrolyte imbalances or deficiencies
  • Low levels of selenium and/or vitamin E
  • Exercising after time off especially if starch intakes were high or not reduced

Myofibrillar Myopathy or MFM in horses

Researcher Professor Stephanie Valberg in the USA has published work describing a subset of horses diagnosed with PSSM2 that exhibit disruption to the muscle fibres and accumulation of a protein called desmin in the muscle. These cases have been defined as MFM. Researchers including Richard Piercy from the Royal Vet College report that they aren’t seeing the same issues in horses in the UK (correct as of Sept 2022). The reasons for this disparity are being explored but if you do refer to advice given by Professor Valberg and are based in the UK or EU then there is some key information to be aware of in relation to her advice which follows in italics below:

  • Forage recommendations: good quality grass or grass-legume hay – grass-legume hays are not commonly used in Great Britain but this suggests it is acceptable to use legume forages such as alfalfa in the bucket feed
  • Bucket feed recommendations:
  • Moderate levels (20-30%) of Non Structural Carbohydrate (NSC) are considered acceptable – in Great Britain a NSC value this high would not be considered moderate and is above levels that would increase the risk of other issues such as gastric ulcers. We would therefore recommend a much lower level of NSC for general health
  • A crude protein value of 12-14% and ensuring cysteine is supplied in the diet as there is some suggestion MFM causes issues in the conversion of methionine to cysteine following exercise – alfalfa is a good way to help achieve this level of crude protein particularly if levels in the forage are low and it is also a good source of cysteine relative to other forages
  • Advocate feeding co-enzyme Q10 – this is not an authorised additive/nutrient in the EU or UK. Importing products containing co-enzyme Q10 is breaking feed legislation rules.

Feeding and management tips for horses tying up:

Much of the advice applies to any of the different disease types. The extent to which you may be willing or able to implement the advice is often determined by the type and level of work the horse is doing. Tying-up in horses, particularly those in racing, can often be career limiting or ending but it is worth noting that research in Scandinavia has shown that Standardbreds could perform racing level exercise on a forage-based ration without compromising performance. This goes to show that it is possible to feed and manage horses, even racehorses, in a way that should help to reduce the risk of muscle problems.


  • Turning out to grass as much as possible is recommended for most horses for their overall health and well-being but particularly for those prone to muscle disorders.
  • There are exceptions such as for horses and ponies with PSSM1 – grass is abundant in sugar and so unlimited access may contribute to the excessive accumulation of glycogen in the muscles.
  • Bringing horses with PSSM1 off grass for some or all of the time may be necessary to help control the amount of sugar they are consuming.
  • As being allowed to move around more than when stabled is also believed to be beneficial, alternative strategies to turning out on grass include using barns, corrals, track systems, woodchip “paddocks”
  • Sports horses with ERS can benefit from being turned out most of the time and can compete at a high level even if they spend most of their time in the paddock. Alex Bragg keeps Quindiva, his 4* horse out as much as possible to help manage her RER. You can hear more on the Horse & Hound podcast from 14th November 2022.


  • Use low sugar forages such as late cut hay or straw which can be really useful for good doers
  • Soaking hay can reduce sugar content but varies according to soak time and temperature in particular
  • A true haylage that has gone through a fermentation process should have a relatively low sugar value as the sugar has been converted to fatty acids. True haylage may be too conditioning for good doers but could be diluted with straw, but makes a good basis to rations for poor doers or those in work. For these groups, better quality forage means less reliance on bucket feed to supply energy which is generally safer.

Bucket Feed

  • Feed diets that are low in starch and high in fibre.
  • Use oil for additional energy or calories – Dengie Alfa A Oil, Performance Fibre, Alfa A Molasses Free and Healthy Tummy are all equivalent to conditioning/competition mixes in terms of their energy content but have much lower levels of starch
  • Many horses and ponies with PSSM are good doers. A lower energy fibre feed with a light oil coating such as Hi-Fi Molasses Free makes a good basis in which to mix a broad spectrum vitamin and mineral supplement or balancer. A high spec supplement or balancer will help to meet their increased requirements to key nutrients such as vitamin E.
  • For every ml of oil in the ration a horse needs 1-1.5 IU of additional vitamin E above their basic requirement
  • Maximum level of oil recommended in the diet is 1ml/kg BW per day. A 500kgs horse receiving 500mls of oil therefore needs an additional 500-750IU of vitamin E. It is important to note that vitamin E is a fat soluble vitamin so is stored in the body.
  • Ensure you are feeding a ‘balanced diet’. This can be provided in the form of a good quality vitamin and mineral supplement or balancer. If feeding less than the manufacturers recommended amount of mix or cubes, remember to ‘top up’ your vitamins and minerals with one of the above.
  • Feed salt daily and electrolytes when necessary. A good electrolyte supplement should contain potassium in addition to sodium chloride

Additional management tips for horses tying up

  • Ensure you warm up and cool down sufficiently – the cool down period helps the horse clear the by-products of exercise from the body
  • Try to keep work intensity and duration consistent when possible
  • Try to exercise daily and little and often if your time is limited – i.e. two or three sessions of 10 or 15 minutes.
  • Keep stress to a minimum
  • If there is any sign of viral infection on the yard, then reduce workload immediately
  • Always seek veterinary advice if your horse has any signs of muscle problems