Iodine is one of 8 ‘micro minerals’ which by definition, means it is needed in very small amounts by the horse. Although only needed in small amounts, 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. However, iodine is one of the few minerals that can be harmful when oversupplied and for which intolerable levels can be reached relatively easily.
Iodine toxicity is more common than deficiency and in most cases, is the result of over feeding high iodine supplements such as seaweed. Even for horses without access to grazing, the recommend ration of compound feed, broad spectrum vitamin and mineral supplement or balancer should easily meet the horse’s iodine requirements without the need for additional supplementation. In fact, because all of the vitamins, minerals and amino acids are naturally occurring (and the levels at which some are provided), a seaweed supplement alone will not provide a balanced diet. In addition, there is no scientific evidence to suggest that seaweed is beneficial joint supplement.
It is estimated that a 500kg horse needs 1-2mg of iodine per day, with the National Research Council (NRC – the accepted institute for standardising animal diets) setting toxic levels at 50mg per day (or 62.5mg for very hard working horses). However, some nutritionists believe that ‘undesirable effects’ or even toxicity can be reached at much lower levels.
Seaweed supplements may typically contain around 835mg of iodine per kilogram and may be recommended at a rate of 50-150g per day for a 500kg horse (iodine levels and recommended rations of seaweed do vary between suppliers). Based on these figures, owners could be providing 42-125mg of iodine per day from seaweed alone. Whilst not all companies selling seaweed recommend rations containing toxic levels of iodine, some nutritionists question whether it is ‘good for horses’ to be oversupplied with a nutrient required in such small amounts.
Excess iodine poses greatest risk to pregnant mares, potentially causing infertility, abortion and due to the high concentration on 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 and can result in goiter, obesity, poor coat condition, lethargy and intolerance to cold.
Seaweed can be fed safely to horses and may have some benefits to offer. However, if you are using a seaweed supplement, first and foremost establish whether or not you reaching toxic levels of iodine (a nutritionist will be able to offer advice on this) and if the answer is yes; remove or reduce your supplement and never provide ‘free access’ or ‘ad-lib’ seaweed.. If not, it may still be worth asking yourself whether you are oversupplying iodine unnecessarily and if any benefits of feeding seaweed could be achieved more appropriately.
