How Much Iodine Does My Horse Need?

Iodine Requirements in Equine Rations

The requirement for iodine in equine rations has been estimated from other species requirements and by evaluating thyroid function. Iodine intake in humans is monitored by testing renal (urine) levels, but normal standards have not been established for equines. Most geographical standards for iodine are mapped based on iodine deficiency in humans; this is unreliable in North America, as the use of iodized salt for human consumption has resulted in a low occurrence of iodine deficiency that does not reflect deficiencies in food sources.

National Research Council (NRC) requirements (1989, page 16) for iodine for horses are estimated to be from 0.1 to 0.6mg/kg diet. That would be 0.9 to 5.4mg/day for a horse eating 20lbs of hay. Researchers at Kentucky Equine Research are currently proposing a range of 1.75 to 3.0mg/day for a 500kg (1100lb) horse depending on work level. Eleanor Kellon, VMD, in Equine Supplements & Nutraceuticals (1998) recommended up to 5mg/day, depending on activity. On the Equine Cushing’s list, Dr. Kellon has suggested 2.5mg iodine/day per 500lbs body weight (or 5.5mg/day for an 1100lb horse) which is consistent with the NRC recommendations. Thyroid panel levels have shown improvement (from below or low normal to mid normal ranges) on this level of supplementation.

NRC estimates the toxic level of iodine to be 5ppm (5mg/kg of diet), or approximately 45-50mg per 20lbs of dry matter fed. Studies cited in Advances in Equine Nutrition I, II & II (Kentucky Equine Research, 2005) indicate that mares produced foals with goiter when fed diets containing 50mg iodine per day, but points out that the analyses of daily iodine intake might not have been accurate and the actual amounts fed may have been higher.

Kelp and other marine products can contain excessively high levels of iodine – a study in the British Journal of Nutrition (1994) showed some edible seaweeds with iodine levels as high as 2,660,000 mcg/kg - or up to 75mg per ounce. An earlier study cited by NRC indicates kelp may contain up to 1850mg iodine per kg (52.5mg/oz). While some quality kelp or other marine plant based supplements are assayed to guarantee safe levels (and these levels are indicated on labels) as these are not regulated there is no requirement to assure safe levels are not exceeded.

Source is made from seaweed, not kelp, and has a guaranteed analysis of iodine (min) 660ppm (18.75mg per ounce). Their recommended feeding rate (one ½ ounce scoop) will provide approximately 9.3mg of iodine, one half scoop will provide approximately 4.7mg of iodine.

The amount of iodine (from potassium iodide) in iodized table salt has a regulated range and will average around 1.72mg iodine per ounce of iodized salt. The range of iodine in iodized table salt can vary from 1.28 to 2.15mg per ounce (Lorrie-Ann Fisher, Lead Research Chemist Morton Salt; private communication). The round number of “2mg iodine per 1 ounce of iodized salt” is used for simplification. [The range is more tightly regulated in Canada than in the US and may vary more in WHO regulated countries.]

For accuracy, any supplement should be fed by weight, not volume, as “kitchen” or other volume measuring devices (teaspoons, tablespoons, scoops, cups, etc) can vary considerably. However, using different scales and a variety of common kitchen measuring teaspoons to weigh three different brands of salt, I found 1 ounce of iodized table salt averaged to approximately 4 teaspoons by volume.

Adding 1-1/2 to 2 ounces salt per 18-22lbs of hay fed per day will balance the typical potassium levels found in most Southwest grown Bermuda hay to a K:Na (potassium to sodium) ratio of between 3:1 and 10:1. [Studies in endurance horses have shown that lowering or even inverting this ratio to as much as 1:2 of K:Na may be needed to avoid exertional rhabdomyolysis/tying-up.]

If iodized salt is fed at the rate of 1-1/2 to 2oz/day, these amounts will provide minimum “insurance” levels of iodine (approximately 2-5mg) for a full size horse eating 18-22lbs of hay a day while remaining well below excessive levels. Testing forage for iodine levels is difficult and expensive. Larry Berger, PhD, University of Illinois, writes in “Trace Minerals for the Animal Nutrition Professional” (Salt Institute 1994/95) that iodized salt be fed in all areas of the United States and that diets containing soybean meal, cottonseed meal and canola meal increase the requirement for iodine.

Another possible concern in many areas of Arizona is the use of Colorado River water blended into drinking water supplies and used as irrigation water. While the overall effect is not yet known, this water has been shown to be contaminated with perchlorate, which inhibits uptake of iodide to the thyroid (Santa Ana Watershed Project Authority, FY2004/05).

If there is any concern about using the upper levels of iodine suggested above, at least the minimum NRC level of 0.1ppm (0.1mg/kg dry matter) or approximately 1mg of iodine per 20lbs of hay should be provided by feeding ½ oz of iodized salt, with the remainder of the horse's salt requirement being provided by an additional 1 to 1-1/2 oz plain salt.

Horses with below or low normal thyroid panel results may benefit from the higher levels of iodine. Serum iodine level testing is available in Europe, but I did not find any US animal lab references for either serum or urine iodine levels. Thyroid function (full thyroid panel) is the standard method currently used to indirectly evaluate iodine status in the US.

References
National Research Council, Nutrient Requirements of Horses, Fifth Revised Edition, National Academy Press, 1989

Advances in Equine Nutrition I, II & III, Kentucky Equine Research, 2005

Eleanor Kellon, VMD, Equine Supplements & Nutraceuticals, Breakthrough Publications, 1998

Ulrich Wehr, Bettina Englschalk, Ellen Kienzle and Walther A. Rambeck, “Iodine Balance in Relation to Iodine Intake in Ponies”, J. Nutr. 132:1767S-1768S, June 2002

Salt Institute articles http://www.saltinstitute.org/24.html

Santa Ana WatershedProjectAuthority
http://www.sawpa.org/documents/perchlorate/AprilPerchlorateReport.pdf