Magnesium
What it does
Magnesium acts as a helper in a large number of enzymatic reactions and is involved in signal transmission between nerve and muscle. It is stored mainly in bone and muscle.
Why it matters
A ration sufficiently supplied with magnesium supports exactly those processes that go unnoticed as long as they work — muscle activity and nerve conduction. They usually become noticeable only when something is missing.
When it becomes critical
Genuine undersupply is rare and most likely on intensively fertilised, species-poor pasture grass. Signs then include over-excitability, muscle tremor and tension, even cramps. A healthy horse tolerates surpluses well; caution applies only when magnesium AND phosphorus are both supplied generously — that combination raises the risk of urinary stones (uroliths).
How much a horse needs
Maintenance is quoted as 52 mg per kilogram of metabolic body mass per day (kg BW^0.75); in everyday terms roughly 4 g daily at 300 kg and 6 g at 600 kg. Work adds a little, lactation and growth a good deal more.
A quiet pacemaker in the metabolism; involved in numerous enzymatic processes and in the interplay of nerves and muscles; stored in bones and muscles.
Cited values
| Type | Value | Unit / context |
|---|---|---|
| Requirement | 4 | g/day (maintenance, 300 kg) |
| Requirement | 6 | g/day (maintenance, 600 kg) |
| Requirement | 53 | mg/kg metabolic body mass/day (maintenance) |
| Toxic from | 8 | g/kg feed DM |
Careful when comparing: the requirement is given **per day**, the upper limit **per kilogram of feed dry matter**. These are different reference bases and cannot be compared directly. The upper limit is also not a target — it is the amount at which harm begins.
“Metabolic body mass” is a calculated figure, not a weight: body weight to the power of 0.75. For 600 kg that is about 121 — the requirement is multiplied by that figure, not by 600. That is why the number looks small even though it isn't.
Possible deficiency signs
- Over-excitability and jumpiness, muscle tension through to cramps (most likely on intensively fertilised, one-sided grass pastures).
Interactions
- Magnesium ↔ Phosphorus (plenty of magnesium plus plenty of phosphorus increases the risk of urinary grit)