I’m concerned about the amount of iodine that seaweed contains. I have been feeding seaweed to my horse as a supplement because I was told it was good for her hooves and her gut, but I’ve read that it contains very high levels of iodine, way over the couple of mg a horse needs per day. I was told that iodine can be toxic in high levels and now I’m worried that I’m feeding toxic levels of iodine.
You are correct that seaweed contains high levels of iodine, but if you feed seaweed from a reputable manufacturer and follow the feeding directions, you will not be feeding toxic levels of iodine. Maximum tolerable levels of iodine are about 50 mg daily for a 500 kg horse, after which the horse could suffer toxic effects.
From a seaweed product that contains 500 mg of iodine per kg, and which is fed at a daily rate of 30 g, your horse will receive 15 mg of iodine, well within the maximum tolerable level of 50 mg per day (based on a 500 kg horse). Pregnant mares have a lower maximum tolerable level, and ideally should not be fed more than 10 mg of iodine daily (based on a 500 kg mare).
Horses require as a minimum, up to 3 mg of iodine per day (based on a 500 kg horse) depending on their workload. Many nutrients are supplied naturally in the diet in much higher quantities than the horse requires, but their bodies are able to extract what they require and/or excrete or otherwise deal with the excesses. In fact it is unusual for a horse to take in just the ‘right’ amount of many of the vitamins and minerals!
Seaweed provides a wide range of benefits to the horse and is reputed to strengthen hooves and improve haircoat, have a beneficial effect on the gut (probably due to its polysaccharide content), support good immune function and help with weight loss. It’s the richest vegetable source of minerals for horses. The range of benefits seaweed provides is worth the high level of iodine it supplies and it’s possible that some horses respond positively to the high level of iodine.