Iodine: It is probably better not to give pregnant mares an iodine supplement. The developing foal has a specific mechanism for selectively drawing iodine from the mar’¨s blood, but, mportantly, no method of excreting any excess.

In the non-pregnant horse, iodine is conserved carefully and excesses are excreted efficiently – once the mare has a fully-stocked store, only tiny daily amounts are needed to keep it that way.

Supplementing iodine can cause a progressive and dangerousaccumulation in the foal. Goitre, musculo-skeletal abnormalities, weakness and even death can develop, although the mare will probably look normal throughout.

Conversely, if the mare were deficient, the foal might be normal, as he would be supported at the expense of the mare. The deficient mare may have retained afterbirth or show reproductive problems.

Electrolytes: High-performance animals, such as racehorses, eventers or endurance horses, require electrolyte replacement afterstrenuous exercise.

Over-supplementation, such as with salt, can be harmful, but small amounts of potassium are helpful, particularly for endurance horses.

Calcium/phosphate: The growing horse needs a diet with a balanced calcium/phosphate ratio to allow normal bone growth. Diets based heavily on wheat, oats, corn or bran often have a high dietary phosphate level.

Abnormal bone growth, sore shins, microfractures, physitis and “bent legs” have all been blamed on abnormal dietary calcium:phosphate ratios. The imbalance can only be established from blood and urine tests and, when confirmed, suitable supplementation with a high calcium source, such as ground limestone (chalk sticks are convenient), will help.

Iron: Deficiency can cause anaemia, but this is rare. The body conserves and recycles iron efficiently and so supplementation is seldom justified.

Copper: Required in minute amounts. Deficiency is rare, but can lead to mild anaemia. Copper deficiency may also be associated with arterial rupture and abnormalities of bone growth (osteochondrosis). Overdosing can be extremely dangerous.

Magnesium:An essential element for nerve function. Some cases of headshaking have been reported to respond to magnesium supplements.

Fluoride: It is thought that equine dental disease has become less common than it was 30 years ago, when fluoride was added to the water supply. Whether this is actually due to the fluoride in thewater is debatable, but as it works for humans, why not for animals? However, there are enormous risks from overdosing with fluoride, which has resulted in life-threatening dental and skeletal diseases.

Beta-carotenes: A horse fed normalrations based on good-quality forage should not be deficient. These are, however, commonly used to supplement brood-mares and mares with reproductive problems, such as stillbirths and failure to cycle normally.

Vitamin B complex: The normal horse has a ready source of almost all B vitamins within the large bowel.

Vitamin B12: Required for red blood cell production. Lack of dietary cobalt can cause a failure of adequate synthesis in the gut and, therefore, deficiencies ofred cell function. Vets are convinced of the benefit of supplementary doses given by injection.