Our experience over many years is that horses exhibiting symptoms commonly associated with PSSM can also be found wherever grass grows, particularly when their diet and management do not adequately address their individual needs.
These horses may show signs that appear to be associated with pasture, seasonal changes or particular types of forage. In some cases, symptoms continue even when green grass is no longer part of the diet, making it important to look at the whole diet, including hay.
Forage analysis can be a useful tool in investigating potential nutritional factors that may be contributing to a horse's symptoms.
Comparing the symptoms
The signs reported in horses described as grass-affected can be remarkably similar to those associated with PSSM and other muscle disorders.
These may include:
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Overall stiffness
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Appearing lazy, reluctant to move or unable to move freely
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Intermittent or shifting lameness
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A tense or uncomfortable abdomen
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Resenting being groomed, saddled or cinched up
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Difficulty training or unwanted behaviours such as bucking, rearing or balking
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Muscle spasms or tremors, particularly around the flanks, neck and shoulders
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Becoming reluctant or unable to move, sometimes stretching out as if attempting to urinate
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General signs of pain or discomfort
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Excessive sweating
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Firm or hard-feeling muscles, particularly over the hindquarters
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Pawing the ground
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Wanting to roll immediately after exercise
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Frequently rubbing the body, particularly the hindquarters, against solid objects
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Difficulty standing comfortably for the farrier
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Standing with the legs tucked under the body or parked out
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Nervous behaviour or excessive spooking
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Difficulty bending through the body, backing up, lunging, cantering or achieving collection under saddle
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Symptoms that come and go, with periods when the horse appears relatively unaffected
In more serious episodes of muscle damage, a horse may develop dark red-brown urine or become unable to stand. These signs require urgent veterinary attention.
It is important to remember that this list is not specific to PSSM or grass-associated problems. Many other conditions can produce similar signs, so symptoms alone cannot establish the underlying cause.
What the comparison tells us
Like horses described as grass-affected, horses diagnosed with PSSM can display a wide variety of symptoms.
In some horses, signs come and go. In others, they may be persistent, severe or potentially life-threatening.
For horses whose symptoms appear to fluctuate with seasonal changes, weather, pasture growth or a move to a fresh paddock, recording these patterns may help identify possible contributing factors.
Our experience has been that many horses described as grass-affected respond well to changes in diet and management. However, there is no one-size-fits-all solution. Each horse has individual requirements, and some need additional investigation and a more tailored approach.
It is also important not to assume that unexplained stiffness, behavioural changes or difficulty moving must be caused by grass or a muscle disorder. Pain, lameness and other medical conditions need to be considered.
A careful assessment of the horse's diet, environment, exercise and overall health can help guide the next steps.
Grass-affected or PSSM: Is there a difference?
When comparing lists of symptoms, there is considerable overlap between those reported in grass-affected horses and those associated with PSSM. It is understandable that horse owners question whether the two might be connected.
Polysaccharide storage myopathy (PSSM) is a muscle disorder involving abnormal glycogen storage or handling within muscle cells. The condition is associated with muscle dysfunction and can cause stiffness, reluctance to move, sweating, muscle tremors and episodes of tying-up.
However, similar symptoms do not necessarily mean that two horses have the same underlying condition.
Types of PSSM
PSSM Type 1 (PSSM1)
PSSM1 is associated with a specific genetic mutation affecting the regulation of glycogen synthesis in muscle. A DNA test can identify this mutation.
Horses carrying the mutation may have different levels of clinical severity. Diet, exercise and other management factors can influence how the condition presents, but management does not remove the underlying genetic mutation.
PSSM Type 2 (PSSM2)
PSSM2 has historically been used to describe horses with muscle symptoms and biopsy findings resembling PSSM but without the PSSM1 mutation. The terminology and understanding of these muscle conditions have evolved, and the underlying causes are not fully understood.
Commercial genetic tests marketed for PSSM2 and related muscle conditions have not been scientifically validated as definitive diagnostic tests. A veterinarian may recommend further investigation, including muscle biopsy where appropriate, to determine what is causing a horse's symptoms.
Could environment and management be contributing factors?
Genes are only part of the picture when it comes to many health conditions. Environmental factors can influence how certain genetic predispositions are expressed and how severely a horse is affected.
This raises an important question for horse owners: when symptoms associated with PSSM overlap so closely with those reported in grass-affected horses, how much of a role might diet, forage and management be playing?
Over the past 15 years, we have worked with horses described as grass-affected and have seen improvements in their symptoms following changes to diet and environment, without those horses necessarily having a PSSM diagnosis.
These experiences reinforce our view that it is worth investigating the horse's nutritional intake and management rather than focusing solely on a label.
However, improvement following dietary or management changes does not establish that the original problem was caused by grass, nor does it rule out an underlying muscle disorder.
The aim should be to identify and address the factors that can be managed while ensuring that other possible causes are not overlooked.
Understanding the role of pasture and forage
Horses have evolved to graze and digest fibrous plant material. However, the nutritional composition of the forage available to domestic horses can differ considerably according to plant species, growing conditions, weather, soil fertility and stage of growth.
In some environments, pasture may be available for much of the year, including periods when plants are growing rapidly. This can create different nutritional challenges for individual horses, particularly those with specific metabolic or dietary requirements.
The problem is not necessarily the presence of green grass itself. Rather, it is important to consider the type of grass, its stage of growth, the amount consumed and how the whole diet meets the horse's individual needs.
This is particularly relevant when a horse's symptoms appear to change with the seasons or after moving to a different paddock.
Don't overlook the hay
Even when a horse has little or no access to green pasture, its hay can still be an important part of the investigation.
Hay varies in its nutritional composition, including sugar, starch and mineral levels. These differences can be influenced by the plant species, growing conditions, maturity at harvest and curing and storage practices.
Where a horse has recurring stiffness, muscle discomfort or other unexplained symptoms, forage analysis may help identify nutritional factors that would otherwise go unnoticed.
Depending on the horse's circumstances, analysis of non-structural carbohydrates (NSC), which include sugars and starch, and relevant mineral levels may be useful. The results should be interpreted in the context of the horse's overall diet and individual requirements.
Dry hay is not automatically low in sugar, just as green grass is not automatically unsuitable for every horse.
Why mineral balance matters
One area worth considering when assessing pasture and forage is mineral intake.
Minerals and electrolytes play important roles in nerve and muscle function, fluid balance and many metabolic processes. The balance between nutrients, as well as their individual amounts, can influence whether a horse's diet meets its requirements.
Pasture mineral composition varies according to soil conditions, plant species, fertiliser use and other environmental factors. Hay and supplementary feeds also contribute to the overall mineral intake.
This does not mean that every grass-affected horse has a mineral imbalance, or that mineral supplementation is automatically required. Excessive supplementation or inappropriate combinations of minerals can create their own problems.
The most useful approach is to assess the whole diet and consider forage analysis and professional guidance where appropriate, rather than adding supplements based on symptoms alone.
For more information about pasture species, growth stages and their nutritional differences, read our article on Grass Choices: Which Is Best for Our Horses?.
When should you seek veterinary advice?
Although investigating diet and management can be valuable, it is important not to assume that all stiffness, lameness or unusual behaviour is grass-related.
A horse that suddenly becomes severely stiff, sweats excessively, develops muscle tremors, is reluctant or unable to move, or passes dark red-brown or coffee-coloured urine needs urgent veterinary assessment. These signs can indicate a serious episode of muscle damage or another medical emergency.
Recurring symptoms also warrant investigation, particularly if they affect the horse's ability to exercise, move comfortably or carry out normal daily activities.
Keeping records of symptoms, pasture changes, forage, supplementary feeds and exercise can help your veterinarian identify patterns and decide whether further testing is appropriate.
A practical approach for every horse
Whether a horse has been diagnosed with PSSM, has another identified condition or is simply showing unexplained symptoms, understanding its diet and environment is an important part of responsible management.
There is no single dietary or management strategy that suits every horse. The aim is to understand the individual, identify potential contributing factors and make appropriate changes while ensuring that underlying medical conditions receive proper attention.
With a better understanding of pasture, forage composition and the horse's individual nutritional requirements, owners can make more informed decisions to support their horse's comfort and wellbeing.
If you would like help reviewing your horse's diet and management, complete our complimentary Horse Questionnaire. We can use the information you provide to better understand your horse's circumstances and discuss appropriate nutritional and management considerations.