Zinc and copper in horses: requirement, hay and ratio

Zinc and copper are among the trace elements that a ration is least likely to cover on its own. Zinc is a building block of keratin formation in skin and hoof horn and is involved in numerous enzymes; a genuine deficiency shows in foals as reduced weight gain, scaly skin changes on the lower limbs and hair loss ¹.

Copper is a component of enzymes in connective tissue, iron and pigment metabolism. The documented deficiency patterns mainly affect foals, in whom copper deficiency is associated with cartilage and skeletal findings ¹. These are exactly the groups for which we give no automatic recommendation – foals, young horses during rearing and pregnant or lactating mares belong in veterinary and breeding advice.

How much does a horse need?

It is worth looking at both reference works here, because they calculate differently. The GfE (German Society of Nutrition Physiology) relates the requirement to metabolic body weight and gives maintenance values of 4 mg zinc and 1 mg copper per kg BW^0.75 per day ². For a 500 kg horse that is around 423 mg zinc and 106 mg copper daily. The Gruber Table based on the GfE confirms this: 505 mg zinc and 120 mg copper for a 600 kg warmblood ³.

The NRC calculates linearly per kilogram of body weight and expresses the requirement mainly as a concentration in the ration: 40 mg zinc and 10 mg copper per kg dry matter ⁴. For the same 500 kg horse this gives 400 mg zinc and 100 mg copper – practically the same figure.

The two only diverge at the edges. For a 200 kg pony the GfE arrives at around 213 mg zinc, the NRC at 160 mg – a difference of one third. We therefore give both values rather than choosing one.

What forage provides

German meadow hay contains in the order of around 20–35 mg zinc and 4–6 mg copper per kg dry matter, with considerable variation by site, soil, growth stage and weather ⁵.

If you work this through for a 500 kg horse eating 10 kg of hay dry matter, a revealing picture emerges: with average hay, around two thirds of the zinc requirement and just under half of the copper requirement are covered. With poor hay it is just over a third and just over a quarter respectively; with very good hay, zinc may be exceeded on paper – copper rarely reaches the requirement mark even then.

The zinc-to-copper ratio – demystified

A zinc-to-copper ratio of about 3:1 to 4:1 is recommended in the ration; a marked excess of zinc can impair copper absorption ¹.

Where this target comes from is less mysterious than it often sounds: it is simply the ratio of the two requirement figures to each other – 4 mg zinc to 1 mg copper for the GfE, 400 to 100 mg for the NRC. If you cover both requirements, you automatically end up there. As an independently derived physiological optimum, the ratio is not proven in horses.

That an excess of zinc interferes with copper absorption, on the other hand, is real – but at a level that has nothing to do with feeding. The classic evidence comes from the area around a zinc smelter, where horses developed generalised osteochondrosis and nephrocalcinosis ⁶. The feed contents reported there are many times higher than anything a supplement could reach.

How much is too much?

Here the bottleneck is not toxicology but feed law. The legal maximum contents apply to the total ration across all sources: 120 mg zinc ⁸ and 25 mg copper per kg complete feed ⁹. What is already in the hay counts too.

For comparison: the scientifically derived tolerance threshold is 500 mg zinc and 250 mg copper per kg dry matter ⁴. Between requirement and biological tolerance there is therefore roughly a factor of 12 to 24 – between requirement and the legal upper limit only a factor of about 2.4 to 2.8.

The typical oversupply case is therefore not an overdosed product either, but the sum: mineralised muesli plus mineral feed plus a special product. Declutter the ration first, then supplement.

Hooves, coat and skin – what you can expect

Hoof horn grows slowly. If someone promises you a better hoof after four weeks, they are not reckoning with biology: the reference studies at the Spanish Riding School ran for 19 months ¹⁰¹¹.

The best-supported single building block for hoof horn is not zinc, but biotin. Pioneering clinical work recommended 10–30 mg daily for at least 6–9 months ¹². And not every defect responds to it: superficial defects respond, while deeper disturbances of the inner keratin layers additionally need bioavailable calcium ¹³.

For zinc itself, the evidence for hooves is thin. The only controlled study involves seven horses and shows denser horn material after six months of zinc methionine, but no clinical endpoints ¹⁴. The honest statement is: correcting an undersupply makes sense; further topping up an already good supply brings nothing according to current knowledge.

Why blood and hair values help little here

Hair analysis does not work for trace elements: in 62 paired hair and liver samples, there was no significant correlation for zinc, and for copper only one too weak to be clinically useful ¹⁵. Blood is difficult too – plasma zinc was not an indicator of liver zinc content, and there is no simple relationship between plasma and liver copper ¹⁶.

What you can reliably determine, on the other hand, is the intake – and that is exactly what you control. For that you need a feed analysis, not a blood sample.