Calcium, phosphorus and magnesium in horses

Calcium, phosphorus and magnesium are the three macrominerals that belong closely together – and the ones about which most half-truths circulate on the yard. Unlike sodium and chloride, which follow their own logic and have their own article, these three come mainly from the same place: forage.

Calcium and phosphorus are by quantity the dominant bone minerals; calcium also controls muscle and nerve excitability and blood clotting, while phosphorus is a building block of energy carriers (ATP) and cell membranes ¹². Magnesium is a cofactor for numerous enzymes and is involved in muscle and nerve function; around 60% of the body's magnesium is held in the skeleton ³².

How much does a horse need?

As with zinc and copper, it is worth looking at both reference works, because they calculate differently. The GfE (German Society of Nutrition Physiology) relates the requirement to metabolic body weight (BW^0.75) and gives maintenance values of 0.164 g calcium, 0.113 g phosphorus and 0.053 g magnesium per kg BW^0.75 per day ⁴⁵. Tabulated in the Gruber Table, this gives the following for common body weights ⁶⁷:

The NRC calculates linearly per kilogram of body weight: 40 mg calcium, 28 mg phosphorus and 15 mg magnesium per kg body weight per day ¹⁸. For a 500 kg horse that is 20 g calcium, 14 g phosphorus and 7.5 g magnesium.

The NRC is higher than the GfE for all three minerals, and the difference grows with body weight – because the NRC scales linearly while the GfE scales metabolically. For magnesium in a 600 kg horse, that means 6.5 g (GfE) versus 9.0 g (NRC), a difference of around 38%. We therefore give both values rather than choosing one or averaging them.

The Ca:P ratio beats the individual value

With calcium and phosphorus, it is not only how much of each is in the ration that counts, but their ratio to each other. A Ca:P ratio of about 1.5:1 to 2:1 in the total ration is recommended; it should not fall below 1:1 ¹⁸. The GfE requirement itself is around 1.45:1 – if you cover both requirements, you automatically end up in the target range ⁵.

On the upper side, horses are surprisingly tolerant as long as the minimum phosphorus requirement remains covered: adult horses tolerate considerably wider ratios ⁸¹. The critical case is the reversed ratio – more phosphorus than calcium. Excess phosphorus inhibits calcium absorption in the gut and can favour skeletal changes ⁸¹. The physiology behind it: in horses, a smaller proportion of phosphorus is truly digested and absorbed than of calcium, so a high phosphorus intake binds calcium and shifts the regulatory loop ⁵².

What forage provides

Grass hay contains in the order of 3–6 g calcium, 2–3.5 g phosphorus and 1–2.5 g magnesium per kg dry matter, with considerable variation by site, soil, cut and botanical composition ⁹. If you work this through for a 500 kg horse eating around 10 kg of hay dry matter, a clear picture emerges: even at the lower end of plausible contents, the maintenance requirement for all three minerals is covered – for calcium with a clear margin.

Grass silage and haylage hardly differ from hay on a dry matter basis; conservation changes the mineral concentration only moderately ⁹. Lucerne (alfalfa), on the other hand, is a legume and very rich in calcium – several times the level of grass hay with a similar phosphorus content – and widens the Ca:P ratio accordingly ²¹.

Magnesium: usually covered, “calming” not proven

Magnesium is the nutrient with the biggest gap between advertising and evidence. In horses that eat sufficient pasture, hay and concentrates, magnesium deficiency is unlikely – forage generally provides enough, and the horse absorbs 30–60% of dietary magnesium ³⁵.

The advertised calming effect is not reliably proven in horses that are adequately supplied. The veterinary reference considers low-dose magnesium supplements probably ineffective on behaviour; genuine sedation only occurs at very high doses that cannot be reached through normal supplementation ³. The evidence is thin: a small study of six geldings that received 10 g magnesium aspartate on top of a ration that already met their requirements found a measured reaction speed reduced from 5.3 to 3.1 m/s – a conference abstract with a very small number of animals, measuring reaction speed, not “calmness” ¹².

How much is too much?

Unlike selenium, iodine, copper, zinc and manganese, there is no maximum content under EU law for calcium, phosphorus and magnesium in complete or complementary feeds – these macrominerals are not on the list of additives requiring authorisation with a set maximum content ¹³¹⁴. The upper limit is physiological, not legal: the NRC gives a tolerable maximum concentration in the total ration of around 2% calcium and 1% phosphorus in the dry matter ¹. For magnesium the safety margin is wide; a moderate excess is excreted well if the kidneys are healthy ³.

The typical imbalance is therefore not a single overdosed product, but the sum: a mineralised muesli plus a mineral feed plus a special product, which incidentally shifts the Ca:P ratio along the way. Declutter the ration first, then supplement.

Why a blood test helps little here

The serum calcium level is tightly regulated by hormones and does not reliably reflect the calcium intake from the ration; it can remain normal despite incorrect supply ²⁸. What can be reliably managed is the intake – and that is established by a feed or hay analysis, not a blood sample ¹⁰¹¹.

Foals and young horses during rearing, as well as pregnant and lactating mares, belong in the hands of a specialist or vet: their requirement rises sharply, and errors in the Ca:P ratio weigh most heavily. The same applies if skeletal or bone problems are suspected and with known kidney or metabolic diseases ⁸².