Although required in relatively small amounts, vitamins and minerals are involved in many essential processes in the body including the transmission of nerve impulses, muscle contractions, immunity, bone health, enzyme composition and energy metabolism. It’s easy to assume that topping up on these small but mighty nutrients can only be of benefit but when it comes to vitamins and minerals, you really can have too much of a good thing and in some cases, over supplementation can even be toxic.

Examples of vitamins & minerals that can be harmful if oversupplied include

  • Vitamin A
  • Vitamin D
  • Copper
  • Zinc
  • Selenium
  • Iron
  • Iodine
  • Sodium


In many cases an oversupply is well tolerated but are there are a few minerals which can be harmful at relatively low levels…



Requirement (500kg horse)                          1-1.25mg/ day

Maximum tolerable level (500kg horse)       10-12.5mg/ day

Recommendation (500kg horse)                  no more than 5mg/ day


Selenium works closely with vitamin E and is an important antioxidant. It also plays as an important role in acquired immunity and supports muscle health. Acute selenium toxicity (from a single high dose) or ‘blind staggers’ can cause blindness, lameness, head pressing, increased heart and respiration rates, colic, diarrhea and even death. Chronic toxicity (a buildup over time), is the most common and causes alopecia (particularly of the mane and tail) abnormal hoof growth and cracking of the hooves around the coronary band which in severe cases, can lead to shedding of the hoof wall. However much lower intakes have been seen to have detrimental effects on hoof health which is why many nutritionists recommended feeding no more than 5mg per day for a 500kg horse.


Did you know? 1000 milligrams (mg) = 1 gram (gm)



Requirement (500kg horse)                           1-2mg/ day

Maximum tolerable level (500kg horse)       approx. 50-60mg/ day


Iodine is essential for the production of thyroxine and tri-iodothyronine (thyroid hormones), which influence numerous aspects of horse health including metabolism, heat regulation and bone development. Although forage is unlikely to meet requirements, feeding the recommended ration of compound feed or balancer easily solves this without the need for further supplementation. Excess iodine poses greatest risk to pregnant mares, potentially causing infertility, abortion and due to the high concentration of iodine in the placenta and milk, weakness and goiter (swelling of the thyroid gland) in foals. In non-pregnant, adult horses, iodine toxicity can cause hypothyroidism; a condition which affects thyroid function/ hormone production resulting in goiter, obesity, poor coat condition, lethargy and intolerance to cold. The good news is that toxicity can easily be avoided and generally only occurs as the result of feeding supplements high in iodine such as seaweed. 


Requirement (500kg horse)                           400-500mg/ day

Maximum tolerable level (500kg horse)         approx. 5-6g/ day


Iron is essential for the transportation of oxygen and as a result, approximately 60% of iron in the body is found in haemoglobin – the protein which gives blood its red colour. However iron is abundant in forage and as result, is oversupplied in the majority of diets. In fact, toxicity is far more common than deficiency and may cause depression, dehydration, diarrhoea, an increased risk of bacterial infections, liver failure and in extreme cases death, particularly as result of over supplementation in foals where absorption is more efficient. Contrary to popular belief, there is no scientific rationale to justify the use of iron based supplements to improve energy levels.


Mineral interactions

Good nutrition is about balance which relies on more than the level of individual nutrients alone.  Some minerals compete for the same absorption sites in the body meaning an excess of one can affect the uptake of another. For example,

  • Excess iron can inhibit the absorption of copper and zinc
  • High levels of zinc can inhibit the absorption of copper – as a result the ratio of zinc: copper in the total diet should be maintained at 3-4:1
  • The ratio of calcium: phosphorus in the total diet should be maintained at 1.5-2:1. High levels of phosphorus affect calcium absorption and may result in bone weakness and/ or deformity



Safety tips

  • Seek advice from a nutritionist before adding supplements to your horse’s diet
  • Feed either the recommended ration of compound feed, a balancer or a broad spectrum vitamin and mineral supplement, not all three
  • Try not to compare feeds based on ‘the numbers’ alone – more does not always equal better, even if toxicity is unlikely
  • Avoid supplements containing iron or iodine (including seaweed)

For more advice on feeding a balanced diet contact the SPILLERS Care-Line



National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press.