Mineral Interactions

Why does it matter if zinc is excessive in the diet or too little compared to copper? Or if there isn’t enough calcium relative to phosphorus?

Trace minerals like copper, zinc, iron and manganese are required in very small amounts but that doesn’t take away their importance in the running of cellular processes in the body. On the other hand, more is definitely not better when it comes to nutrients as excessive amounts of some minerals will create problems by interfering with other minerals.

For example, the 2007 Nutrient Requirements of Horses from the National Research Council (NRC) has set the requirements per day for a 500 kg mature horse in light work at 100 mg for copper and 400 mg for zinc. That is per day, many pastures and mixed feeds provide far less than this in copper and usually zinc. However, it is not just the total amount per day that counts, the ratio of copper to zinc counts just as much if not more. The recommendation is to feed copper to zinc ideally at the ratio of 1:3. So if your horse is getting 600 mg of zinc per day in his diet then the copper amount ideally should be boosted to 200 mg.

Why is this necessary? Because in many cases minerals compete with each other.

Some examples –

  • Too much zinc can interfere with copper, causing copper deficiency symptoms and too much copper can cause zinc deficiency

  • Too much copper can interfere with selenium absorption; this has been shown in studies with other animals

  • Sulfur can prevent copper absorption; shown in cattle studies

  • Molybdenum can prevent copper absorption

  • High iron intake is known to reduce zinc levels

  • Too much phosphorus will inhibit calcium absorption

  • Too much calcium will inhibit phosphorus absorption

  • Too much manganese interferes with phosphorus absorption

  • Too much manganese has been suggested to cause intestinal mineral competition in other species

  • In other animals, it has been shown that excessive levels of nutrients can interfere with selenium absorption.

Furthermore, adverse mineral interactions can lead to:

  • Too little calcium will cause a horse to get 'big head disease' called Osteodystrophia fibrosa, a deformity of the facial bones caused by calcium loss. Other signs before the 'big head' symptom may show up include intermittent lameness, loose teeth, stunted growth in youngsters, ruptured tendons and increased chances of fractures. If the horse has the swollen facial bones then he may well have trouble swallowing and chewing and obstruction of the nasal passages. Not a happy camper. When there is too little calcium in the diet the parathyroid gland releases parathyroid hormone. Since the blood calcium level is tightly regulated, this hormone dissolves the calcium in the bones so it can go into the blood to keep blood calcium levels at the correct level for heart and muscle function. The facial bones are replaced with fibrous tissue which appears swollen compared to bone. High phosphorus intakes significantly depress calcium absorption.

Big Head

  • Insufficient amounts of magnesium can cause irritability, twitching, spasm and hypersensitivity. Many calming supplements contain magnesium which explains why they can help. However, they are often expensive and contain other ingredients you don't always need. Magnesium can be supplemented in a more cost effective way than this.

  • Excessive calcium will also cause irritability, twitching, spasm and hypersensitivity. That is because the calcium:magnesium ratio is too high and extra magnesium will help. But if your horse is already getting too much calcium in his diet it may be better to change his feed to reduce the calcium. This will effectively lower the calcium:magnesium ratio.

  • Inadequate copper can cause anaemia. This condition is often misdiagnosed as a iron deficiency, iron metabolism requires copper containing enzymes so if there is a lack of copper then one of the results is less haemoglobin in the blood.

  • Inadequate copper and/or zinc will produce faded coats on horses, known as 'sunbleaching'. If your horse has a dull or washed out coat then insufficient copper in the case of buckskins and chestnuts is most likely the reason and in the case of black, brown and grey horses, copper and zinc is needed for pigment production. A lot of people will rug their horses to stop the 'sunbleaching' but I prefer to see what mineral deficiencies are present and correct them in the right proportions.

To provide an optimised diet to your horse all the major minerals (calcium, phosphorus and magnesium) and trace minerals (iron, copper, zinc and manganese) need to be in balance. And it is easier than you may think. A feeding plan from Balanced Equine provides recommendations aiming as close as possible to a balanced intake based on forage data. The feeding plan provides a ‘recipe’ for making your own custom mineral supplement. Easy to make and the ingredients are not expensive.

You can learn how to balance your horse’s intake by enrolling in NRCPlus with Dr Eleanor Kellon VMD.

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