This article has not yet been professionally approved. The information is based on the sources listed below.
Magnesium for horses: requirements, hay and what supplementing achieves
How much magnesium a horse needs per kg of body weight, how much of it is already in the hay, when supplementing achieves nothing and what the evidence says about the advertised calming effect.
The short answer
According to the GfE (German Society of Nutrition Physiology), an adult horse needs 53 mg of magnesium per kg of metabolic body weight for maintenance – around 5.6 g a day at 500 kg. The NRC uses 15 mg per kg of body weight, i.e. 7.5 g 12. In a Bavarian study, grass hay contained an average of 1.8 g per kg of dry matter 3. So only a calculation with your hay shows whether supplementing is necessary.
How much magnesium does a horse need per kg of body weight?
The GfE gives the maintenance requirement for magnesium as 53 mg per kg of metabolic body weight (BW^0.75) per day 45. Because requirements do not rise linearly with weight, a horse twice as heavy does not need twice as much. The Gruber Table calculates this for common weights 1.
The NRC, by contrast, calculates linearly with 15 mg per kg of body weight per day 26. The heavier the horse, the further apart the two references are. That is why we give both values rather than choosing one.
| Body weight | GfE | NRC |
|---|---|---|
| 200 kg | 2.8 g | 3.0 g |
| 400 kg | 4.7 g | 6.0 g |
| 500 kg | 5.6 g | 7.5 g |
| 600 kg | 6.5 g | 9.0 g |
| 800 kg | 7.9 g | 12.0 g |
Magnesium maintenance requirement per day. GfE values from the Gruber Table; NRC column calculated by us from 15 mg per kg of body weight.
How reliable are these requirement values?
Less reliable than the tables suggest. A meta-analysis of 42 studies found lower endogenous faecal magnesium losses in ponies than in horses. Calculated factorially, ponies needed about half and horses 1.9 times the comparison recommendation of Kienzle and Burger; on the overall average, the result was in line with previous values 7.
The references do not agree on work either. The Gruber Table lists the same magnesium values for light to heavy work as for maintenance 1. The veterinary reference, by contrast, states a possibly doubled requirement for growing, lactating and working horses 8. The table values are therefore a guide for the ration calculation, not a sharp limit.
How much magnesium is in hay?
In a study of hay samples from horse yards in Upper Bavaria, the mean was 1.8 g of magnesium per kg of dry matter. The range was 0.7 to 5.3 g. First cuts contained an average of 1.5 g, later cuts 2.6 g per kg of dry matter 3. According to the veterinary reference, samples of predominantly grass hay from a commercial feed laboratory contained 0.15–0.31% magnesium 2.
A freely chosen worked example, not a real sample: a 500 kg horse eats 10 kg of hay with 85% dry matter, i.e. 8.5 kg of dry matter. At the mean of 1.8 g/kg, that is around 15.3 g of magnesium a day. At the lowest measured value of 0.7 g/kg, it would be just under 6.0 g. That would still be above the GfE value of 5.6 g but below the NRC value of 7.5 g 312.
The example shows both: with enough hay, the requirement is often covered. With a small amount of hay, a poor first cut or a hay replacer, it can get tighter. Only an analysis of your batch tells you which applies to you.
How can you tell if there is a magnesium deficiency?
A diet-related magnesium deficiency is very rare in horses. As exceptions, the veterinary reference names extreme situations in which reduced intake and increased requirements coincide, such as long transports of lactating mares without feed or prolonged low-magnesium infusion therapy 8.
A magnesium deficiency therefore cannot be inferred from spookiness, nervousness or tension. Your vet can assess magnesium status, among other things, through the fractional excretion of magnesium in the urine; in healthy horses on grass hay it is 15–35%, and values below 6% indicate insufficient intake 8.
Does magnesium calm a nervous horse?
Magnesium is often advertised for calming; this very use is the least well supported by evidence. The veterinary reference considers it unlikely that low-dose magnesium supplements influence behaviour. For the commonly recommended 3–10 g, it expects no notable clinical benefit 8.
The frequently cited study is a conference abstract with six geldings. They were given 10 g of magnesium aspartate on top of a ration that already met their requirements; what was measured was reaction speed, not calmness or rideability 9. No calming effect in everyday yard life follows from this.
The same check applies to combination products, such as magnesium with tryptophan: factor the magnesium content into the ration using the daily amount, and do not read calming claims as evidence. Feed must not carry disease-related healing claims 10.
Clarify the cause of nervousness with your vet and in training rather than masking it with a supplement.
How do you calculate a magnesium dose correctly?
The packaging often states the amount of a compound, but what counts for the requirement is the magnesium it contains. The veterinary reference shows how big the difference is: the entire maintenance requirement of a 500 kg horse is supplied by about 16 g of magnesium oxide, 32 g of magnesium carbonate or 47 g of magnesium sulphate 8. This is a conversion example, not a feeding recommendation, as hay already supplies magnesium.
A freely chosen example, not a product declaration: if a complementary feed contains 100 g of magnesium per kg and you feed 30 g daily, that is 0.03 kg × 100 g/kg = 3 g of magnesium. The analytical constituents in the declaration give you the content 10. Add this contribution to that of hay, concentrates and mineral feed before comparing it with the requirement.
- Read the magnesium content from the analytical constituents, not the amount of the compound.
- Multiply by the daily amount actually fed.
- Add up the contributions of all products and the forage.
- Only then compare with your horse's requirement.
Can too much magnesium be harmful?
With healthy kidney function, excess magnesium is excreted well via the kidneys. That does not mean more is without consequences: the veterinary reference points out that excessive long-term supplementation can increase the risk of intestinal stones (enteroliths) 8.
An excess can also arise without a single high-dose product, namely from the sum of mineral feed, mineralised muesli and an additional magnesium product. So first check what your ration already supplies, and only supplement a calculated gap.
Key takeaways
Frequently asked questions
How much magnesium does a horse need per day?
For an adult 500 kg horse at maintenance, the Gruber Table (a German feed table) based on the GfE gives 5.6 g daily, and 6.5 g for 600 kg 1. The NRC uses 15 mg per kg of body weight; that gives 7.5 g and 9 g respectively 2. Both values refer to total intake from all feeds, not the amount of a supplement.
Will magnesium calm my horse?
There is no robust evidence for this. The veterinary reference considers it unlikely that low-dose magnesium supplements influence behaviour and expects no notable benefit from 3–10 g 8. The frequently cited study included six horses and measured reaction speed, not calmness 9.
How can I tell if my horse has a magnesium deficiency?
Not from its behaviour. A dietary deficiency is very rare and arises mainly when reduced intake and increased requirements coincide 8. Your vet can assess magnesium status, among other things, through urinary magnesium excretion 8. Restlessness or muscle tremors should be checked by your vet, not treated with a supplement.
Can a horse get too much magnesium?
With healthy kidneys, excess is excreted well. However, the veterinary reference points out that excessive long-term supplementation can increase the risk of intestinal stones (enteroliths) 8. Giving more than the ration needs is therefore not a safety buffer.
Sources & studies
The content is based on the references listed below. It does not replace individual veterinary or feeding advice.
Show list of sources (11)
- Bayerische Landesanstalt für Landwirtschaft (LfL): Gruber Tabelle zur PferdefütterungPractical reference table with requirement and feed values based on the GfE recommendations; the basis of many ration calculations in German-speaking countries. Chapter 9.3, Table 39: most common poisonous plants for horses with habitat, toxicity and risk level (after Kamphues et al. 2014)
- Merck Veterinary Manual: Nutritional Requirements of Horses and Other EquidsSummary of the NRC recommendations: water requirement 5 litres per 100 kg body mass per day at maintenance, sweat losses of around 3.1 g sodium, 5.3 g chloride and 1.6 g potassium per litre, salt recommendation 1.6–1.8 g per kg feed dry matter at a maximum of 6% of the ration; from a 50% forage share potassium is adequately covered. Also covers the selenium requirement (0.1 mg/kg feed dry matter or at least 1 mg/day for a 500 kg horse, selenium as a component of glutathione peroxidase together with vitamin E) and the strong location-dependent variation of selenium in forage (grass forage on average 0.194 ppm, range 0–1.2 ppm); some regions are markedly low in selenium and may need supplementation. Also gives the iodine requirement (0.35 mg/kg feed dry matter, mares in late pregnancy 0.4), iodine poisoning of pregnant mares from around 40 mg/day, documented goitre cases in mares and their foals associated with large amounts of dried seaweed (kelp), that iodised salt easily covers the requirement, and that iodine supplementation is usually unnecessary except in severely iodine-deficient regions
- Möllmann F (2007): Analysen und Abschätzung des Mineralstoffgehaltes in Heuproben aus oberbayerischen Pferdehaltungsbetrieben. Diss. med. vet., LMU MünchenGrass hay samples from horse yards in Upper Bavaria; empirical database for the mineral contents (Ca, P, Mg etc.) of German grass hay, with wide variation by location, soil and cut. Open access.
- Kienzle, Pankratz & Zeyner (2023) – Kompendium zur Rationsberechnung für PferdeAuthor group Pferdewissen (Kienzle/Pankratz, LMU Munich; Zeyner, University of Halle-Wittenberg); self-published, Oberschleißheim, ISBN 978-3-00-075668-9. No link – self-published, not freely accessible.
- GfE (2014) – Empfehlungen zur Energie- und Nährstoffversorgung von PferdenSpecialist reference in German-speaking countries (GfE, German Society of Nutrition Physiology), Committee for Requirement Standards
- NRC – Nutrient Requirements of Horses, 6th ed.International reference for the nutrient requirements of horses (National Research Council)
- Maier I, Kienzle E (2024): A Meta-Analysis on Quantitative Calcium, Phosphorus and Magnesium Metabolism in Horses and Ponies. Animals 14(19):2765 (LMU München)Meta-analysis: sweat losses of Ca, P and Mg are so small that no additional work-related requirement is applied. For Mg it calculates the replacement of endogenous faecal losses and separates by weight (horses > 300 kg: 37, ponies < 300 kg: 11 mg/kg metabolic body mass; a pony that was temporarily heavier remained in the pony group); the comparison factors (horses around 1.9×, ponies around ½) refer to Kienzle & Burger (20 mg/kg), not to the GfE
- Merck Veterinary Manual: Magnesium as an Equine Dietary SupplementStandard veterinary reference work: absorption of dietary magnesium 30–60%; grazing or hay-fed horses are rarely Mg-deficient; low-dose Mg supplements probably do not affect behaviour, sedation only occurs at very high doses.
- Dodd J, Doran G, Harris P, Noble G (2015): Magnesium aspartate supplementation and reaction speed response in horses. Journal of Equine Veterinary Science 35(5):401–402Conference abstract, 6 Standardbred geldings: 10 g magnesium aspartate in addition to a ration that already met requirements shortened a measured reaction speed from 5.3 to 3.1 m/s. Very small number of animals, endpoint reaction speed (not “calmness”) — weak evidence.
- Verordnung (EG) Nr. 767/2009Placing on the market and use of feed, labelling and claims; definitions of complete feed (Art. 3(2)(i)) and complementary feed (point (j)), content of feed additives (Art. 8), feeding instructions (Annex II No. 4). Link to the current EUR-Lex consolidated version (as of 26 December 2018).
- FN – Anti-Doping und Medikation (ADMR-Suchmaschine)Official database of the FN (German Equestrian Federation) on substances and withdrawal periods in competitive sport