Seaweed supplements have become a popular choice amongst horse owners and in some cases, are even fed ad lib or ‘free choice’. However with the potential to do more harm than good, is seaweed a delicacy best reserved for our Chinese takeaway?



Are there any benefits?

Many people claim seaweed is an effective broad spectrum, joint and immune supplement but can these claims really be justified? All of the vitamins and minerals in seaweed are naturally occurring and whilst this may sound like an added health benefit, in reality it means levels are variable making it difficult to determine your horse’s intake. Many supplement companies recommend a daily feeding rate of 50g for a 500kg horse which is unlikely to provide a balanced diet. The table below compares the level of 3 minerals (all of which are typically low in UK forage) provided by the recommended ration of one seaweed supplement currently marketed for horses and a traditional feed balancer. The figures in brackets refer to the % of the daily requirement for a 500kg horse in light work.




There is no scientific evidence to suggest that seaweed is an effective joint supplement which is true for the majority of equine supplements. However even if they prove to be ineffective, iconic ingredients such as glucosamine, chondroitin and MSM will not cause any harm which may not be case for seaweed…


Seaweed dangers

Seaweed is naturally high in iodine, one of the few minerals which can be harmful if oversupplied. Although iodine plays a number of essential roles, horses require just 1-2mg per day with toxicity set approximately 50mg per day (NRC, 2007). However some nutritionists believe that ‘undesirable effects’ may be found at much lower levels. UK forage is typically low in iodine but the horse’s requirements can easily be met by feeding the recommended ration of compound feed or balancer. In fact, toxicity in horses is far more common than deficiency and is generally only reached as a result of feeding supplements high in iodine such as seaweed.



Iodine toxicity

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.


How much seaweed is too much?

Unfortunately this million dollar question is very difficult to answer. The majority of seaweed supplements don’t come with a nutritional analysis and even when they do, it is likely to be an estimate or ‘typical analysis’ and therefore not necessarily representative of what’s in your tub. Several studies have looked at the iodine content of commercially available seaweed supplements intended for human consumption and have reported highly variable results. Teas et al (2014) looked at 3 species of seaweed and reported average levels of 36.9mg/ kg, 139.7mg/kg and 2523.3mg/ kg. However collectively levels ranged from 29.3-4,921mg/kg and in another study levels ranged from as little 5mg/ kg to 12,000mg/kg (Holdt and Kraan, 2011). To help put this variability into perspective, you would need to 10kg of seaweed containing 5mg iodine/kg to reach toxicity vs. just 20g of a supplement containing 2523mg/ kg.



Feeding a seaweed supplement will not automatically result in toxicity and in fact, many horses are fed seaweed without any ill effects. However due to the variability in iodine concentration, it is very difficult to determine whether or not you run the risk of reaching toxicity. Our advice? Do not feed seaweed supplements to foals or pregnant/ lactating mares and for all other adult horses, consider whether the reported ‘benefits’ could be better achieved with a more appropriate feed, balancer or supplement. If seaweed is still your supplement of choice, never feed it ad lib and always seek advice from a nutritionist before introducing it to your horse’s diet.


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