Seaweed supplements have long been promoted as an effective vitamin and mineral supplement for horses, a source of amino acids and as a joint supplement. However due to the high iodine content, feeding seaweed could potentially have harmful effects.
Iodine is a ‘micro mineral’ 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 also 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 dangerous 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 recommended 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, all of the vitamins, minerals and amino acids are naturally occurring and iodine aside, may be found in relatively low levels meaning that seaweed supplement alone will not provide balanced diet. In addition, there is no scientific evidence to suggest that seaweed is beneficial joint supplement.
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 may cause hypothyroidism; a condition which affects thyroid function and hormone production and may 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, it’s crucial that you establish whether or not you reaching toxic levels of iodine and under no circumstances should you provide your horse or pony with ‘free access’ to seaweed supplements.