Magnesium, Grass and the “Spring Crazies”: Does Magnesium Really Calm Horses?
Every spring and autumn, as lush green pasture begins to grow, horse owners start asking the same question:
**”Should I feed magnesium to calm my horse?”**
It’s one of the most commonly recommended supplements for spooky, excitable horses, and every year feed stores and social media are filled with claims that magnesium is the answer to the dreaded “grass crazies.”
But does the science actually support those claims?
The answer is… **it’s not that simple.**
Magnesium Is Essential
Magnesium is an essential major mineral involved in more than 300 enzyme systems throughout the body.
It plays important roles in:
* Muscle contraction and relaxation
* Nerve transmission
* Energy metabolism
* Bone development
* Normal heart function
* Maintaining electrolyte balance
Around 60% of the body’s magnesium is stored in bone, while much of the remainder is found within muscles and soft tissues. Less than 1% circulates in the bloodstream, making blood tests a poor indicator of the body’s total magnesium status.
What Happens if a Horse Is Magnesium Deficient?
True magnesium deficiency can affect both muscles and the nervous system.
Signs may include:
* Muscle tremors
* Muscle tightness
* Increased sensitivity to touch
* Nervousness
* Poor coordination (ataxia)
* Increased excitability
Magnesium is required for the enzyme acetylcholinesterase, which helps regulate nerve signals by breaking down the neurotransmitter acetylcholine. When magnesium is deficient, acetylcholine may accumulate, increasing neuromuscular excitability.
Importantly, these signs can look very different from the behavioural changes caused by excessive dietary sugars or ryegrass toxins.
Not Every “Spooky” Horse Is Magnesium Deficient
This is one of the biggest misconceptions I see.
Just because a horse becomes more reactive during periods of rapid pasture growth does **not** automatically mean it requires more magnesium.
Spring and autumn behaviour can be influenced by many factors, including:
* Increased energy intake from lush pasture
* Longer daylight hours
* Hormonal changes
* High potassium levels in pasture
* Ryegrass endophyte toxins (such as lolitrem B)
* Environmental excitement
* Training and management
Different causes require different solutions.
Simply feeding more magnesium will not solve every behavioural problem.
Is Magnesium Deficiency Common?
Fortunately, true dietary magnesium deficiency appears to be relatively uncommon in horses grazing good-quality pasture in New Zealand.
Most pastures contain adequate magnesium to meet a horse’s basic requirements.
However, magnesium absorption can be influenced by the overall diet.
The Potassium Connection
New Zealand pastures are often naturally high in potassium, particularly during periods of rapid growth.
High dietary potassium may reduce magnesium absorption within the digestive tract by competing for uptake.
This doesn’t necessarily create a magnesium deficiency, but it may reduce the amount available to the horse under certain conditions.
This is one reason why balancing the **entire diet** is much more important than simply adding one mineral.
Grass, Nitrates and Magnesium
Rapidly growing pasture may also accumulate nitrates, particularly during cool, cloudy conditions or when plants are very young.
When nitrate intake is high, the body must safely eliminate it. This process uses positively charged minerals such as sodium, calcium and magnesium.
If dietary sodium intake is low—which is common in horses grazing pasture—greater reliance may be placed on calcium and magnesium during nitrate metabolism.
Although this may increase magnesium demand under certain conditions, it does not automatically mean every horse grazing lush pasture requires magnesium supplementation.
Can You Test for Magnesium Deficiency?
Unfortunately, it isn’t easy.
Because almost all of the body’s magnesium is stored in bone and muscle, blood magnesium concentrations often remain within the normal range even when body stores are reduced.
Muscle biopsy provides a more accurate assessment but is rarely practical.
A much more useful approach is evaluating the entire diet through pasture analysis, forage analysis and ration balancing.
This identifies both magnesium intake and other minerals that may be influencing absorption.
Does Magnesium Actually Calm Horses?
This is where things become interesting.
Despite magnesium’s popularity, there is surprisingly little scientific evidence showing that supplementing magnesium consistently calms healthy horses already receiving adequate dietary magnesium.
One often-cited study found that supplementing Standardbred horses with magnesium aspartate reduced reaction speed when compared with pre-supplementation measurements.
However:
* only six horses were studied,
* supplementation was short-term,
* and the horses were already receiving diets that met recommended magnesium requirements.
While the findings were encouraging, they cannot be applied to every horse.
Numerous horse owners also report improvements after feeding magnesium.
Whether these improvements result from correcting a mild deficiency, altering the overall diet, changes in pasture access, improved management or simply natural seasonal variation remains difficult to determine.
How Much Magnesium Does a Horse Need?
According to the National Research Council (NRC), a 500 kg horse in light work requires approximately **10 grams of magnesium per day** from the total diet.
That total includes:
* Pasture
* Hay
* Chaff
* Commercial feeds
* Mineral supplements
It does **not** mean adding 10 grams of supplemental magnesium on top of everything else.
## More Is Not Better
One of the most common problems I encounter is over-supplementation.
Many horses are receiving magnesium from multiple products simultaneously.
When pasture, forage, commercial feeds and several magnesium supplements are added together, total intake may exceed 300–400% of requirements.
Although healthy horses excrete excess magnesium in urine, excessive supplementation may:
* interfere with calcium and phosphorus balance,
* disrupt mineral ratios,
* affect intestinal function,
* increase unnecessary expense,
* and provide no additional behavioural benefit.
More supplement does not necessarily mean a calmer horse.
Could the Improvement Be Due to Something Else?
Quite often, yes. When owners begin feeding a magnesium supplement they also frequently:
* remove horses from lush pasture,
* introduce more hay,
* reduce grain intake,
* improve overall mineral balance,
* add sodium,
* add Vitamin B1,
* improve training and management.
Any—or all—of these changes may contribute to improved behaviour. This is why it is difficult to attribute improvements solely to magnesium.
Practical Recommendations
If your horse becomes more reactive during spring or autumn:
* Assess the whole diet rather than focusing on one mineral.
* Ensure sodium intake is adequate.
* Maximise good-quality forage.
* Limit excessive starch and sugar intake where appropriate.
* Consider pasture and hay analysis.
* Review training, handling and environmental factors.
* If magnesium supplementation is appropriate, begin before periods of rapid pasture growth rather than waiting until behavioural issues become severe.
The Bottom Line
Magnesium is an essential mineral that plays a critical role in muscle and nerve function, and correcting a genuine deficiency is important for your horse’s health.
However, most pasture-fed horses are not truly magnesium deficient, and the scientific evidence supporting magnesium supplementation as a universal calming supplement remains limited.
Rather than assuming every excitable horse needs more magnesium, consider the bigger picture.
Balanced nutrition, appropriate pasture management, adequate sodium intake, correct training and understanding the underlying cause of behavioural changes are far more likely to produce lasting results than relying on a single supplement.
Good nutrition is about balance—not simply feeding more minerals.
Resources:
Dodd J.A., D. G. (2015). Magnesium aspartate supplementation and reaction speed response in horses. Journal of Equine Veterinary Science 35, 401-402.
Harrington D, Walsh J. Equine magnesium supplements: Evaluation of magnesium oxide, magnesium sulphate and magnesium carbonate in foals fed purified diets . Equine Vet J 1980.
Pearson, W., and J. MacNicol. 2017, Acute effects of a single-dose nutritional product on stress response and task completion in horses. Journal of Equine Veterinary Science 51:86-91.
