Iodine - Too Much or Not Enough?

In the latest issue of a well known equine publication, an article on thyroid dysfunction suggests that "...most equine diets of hay and pasture grass (other than in the Pacific Northwest) contain far more than the daily required level of iodine...", and, in a sidebar, cautions that supplementation of iodine could worsen a suspected thyroid issue. Also overlooked here is the small but important role selenium plays in thyroid function.

Iodine deficient soils in the United States

Let's look at some of the facts about iodine.

The National Research Council (NRC), based on the best evidence availablle, suggests a dietary requirement of iodine for horses as a range and recommends using the average of that range as a minimum target.

Suggested range 0.1-0.6 mg Iodine per kilogram of drymatter (DM)
Average of range (0.1-0.6 mg I/kg DM) 0.35 mg I/kg DM

This can also be expressed as a weight-based requirement, presuming the horse has a daily feed intake at 2% of its body weight (BW), which works out similar to the above average.

Maintenance requirement 0.007 mg/kg BW
For a 1000 lb horse,  455 kg x 0.007 = 3.18 mg Iodine/day

For a 1000 lb (455 kg) horse taking in 2% of BW (9.1 kg DM) using the "range" above

low   0.10 * 9.1 = 0.91 mg Iodine (NRC questions if this concentration is sufficient)
avg  0.35 * 9.1 = 3.185 mg Iodine
high  0.60 * 9.1 = 5.46 mg Iodine

(Pregnant or lactaing broodmares have a higher recommended requirement of 0.4 mg I/kg DM.)

The maximum tolerable level for iodine has been set at 5 mg I/kg DM, or 45.5 mg for a 1,000 lb (455 kg) horse.  In a few instances (AZ horses on CAP water, high nitrates in hay, which I now always check in Southwestern hays and other grass hays with high protein levels) I will double the NRC recommendation but otherwise stick with the mid to high range.

From NRC - "Iodine toxicity [seen as goiter] usually results only when iodine is oversupplemented or when animals are receiving feeds containing unusually high amounts of iodine such as some types of seaweed." but toxicity in mares receiving as little as 40 mg a day has been reported.

Iodine concentrations in most common feedstuffs range from 0 to 2 mg/kg DM (equivalent to 0 to 18 mg in 20 lbs or 9.1 kg of hay) depending on the iodine concentration in the soil in which they are grown.  As with other minerals, many conditions can affect forage uptake of iodine from the soil and different farming practices may deplete or concentrate available levels.

Depending on free-choice intake of iodized salt to supply iodine requirements has been shown to result in iodine deficiency in pregnant mares resulting in leg abnormalities in foals, and even when iodized salt is directly added to the feed, the amount may not meed NRC requirements.

Without going overboard, ensuring an Iodine intake level equal to the standard NRC recommendation of 0.35 ppm up to the "high" range of 0.6 ppm might also provide insurance against uptake of the [very] low levels of I-131 detected by Environmental Protection Agency (EPA) monitoring and other environmental "insults".

It is unlikely that using even the higher suggested range would tip your horse over the maximum tolerable level even if your forage contains a fairly high level of iodine. But we should also keep in mind that there is little direct equine research on iodine; without this it's difficult to consider levels higher than those which have been documented to cause goiter in studied horses - 5 mg I/kg DM, or 45.5 mg for a 1,000 lb (455 kg) horse.

Iodine testing for hay and forage is not readily available at most forage labs in the US; when it is the cost may be prohibitive.  For example, at Cumberland Valley Analytical Lab, the price for their Iodine test is $75. On the other hand, if your horse is experiencing issues you feel may be related to excessive iodine intake, the cost may be a worthwhile investment.

Iodine serum level testing (Iodine Total and Iodine Inorganic) is offered at the Michigan State Diagnostic Center. From their Iodine Testing document :

Total serum iodine and particularly serum inorganic iodine reflect the current dietary iodine consumption. Serum inorganic iodine is a good short-term measure of iodine consumption and will reflect excessive iodine supplementation, when present.

Often overlooked when considering possible thyroid dysfunction is selenium deficiency - selenium is necessary for conversion of the inactive hormone T4 to the active thyroid hormone T3 and selenium deficiency together with high iodine intake may result in low T4 levels and other thyroid issues.
http://www.ncbi.nlm.nih.gov/pubmed/9187638  Selenium level testing in whole blood in horses is readily available and also easily included in forage testing (generally under $40).

In summary, iodine is but one element of an overall mineral balanced equine diet. As with any other mineral or supplement, targeting just one nutrient without taking into account how all nutrients interact with one another is not likely to produce the desired results.

Don't throw away the iodized salt or iodine supplement but do make sure that protein, calories, major minerals and other trace minerals are balanced and appropriate for the horse's age, weight, work and reproductive status.  Check that any kelp or seaweed products you use contain guaranteed levels of iodine. "Sea salt", despite the reassuring name, often contains only minute traces of iodine and can't be relied on as an iodine source.

The next time your vet draws blood consider having her check a selenium level - especially if it's been suggested to add a "thyroid" supplement. It makes little sense to use a drug to correct a basic trace mineral deficiency - adding the NRC recommended levels of iodine and selenium may help normalize thyroid function test results.

Warm regards,

Patti in monsoon flooded Vail Az
Don't forget the salt!


Links
Nutrient Requirements of Horses, Sixth Revised Edition  (NRC 2007) Pages 91-92.
http://www.nap.edu/catalog.php?record_id=11653

My previous post about iodine - not much has changed
http://desertequinebalance.blogspot.com/2006/10/iodine-requirement-in-equine-rations.html
and thyroid (including links to Dr. Eleanor Kellon's "Understanding Thyroid" article in Holistic Horse)
http://desertequinebalance.blogspot.com/2013/02/understanding-thyroid.html

Cumberland Valley Analytical Services
http://www.foragelab.com/Lab-Services/Forage-and-Feed/Price-List

Iodine Deficient Soils Map
http://www.goatworld.com/articles/nutrition/iodine.shtml

Michigan State Diagnostic Center for Population and Animal Health
http://www.animalhealth.msu.edu/Bin/Catalog.exe?Action=Test&PAGE=12&Id=2236
http://www.animalhealth.msu.edu/Bin/Catalog.exe?Action=Test&PAGE=12&Id=2920
http://www.animalhealth.msu.edu/Sections/Nutrition/WEBCD.Nutr.REF.003.pdf

Responsible Nutrition

I'm often asked what I think of "this supplement" or a particular feed.

As most people I discuss equine (and human) nutrition with know, I don't believe in "magic bullets". I am adamantly against fear-based promotion (if you don't do this, your horse will die and you are a bad horse owner).

I do subscribe to some basic principles

• You are responsible for what goes into your horse's mouth. This doesn't mean you need to have a college degree in nutrition, but you should have an understanding of what your horse needs and how to supply it.
• Most commercial supplements/feeds are based on NRC guidelines and are amazingly similar. However, they can't address deficiencies/excesses in the specific forage base you are feeding - there is no "one size fits all".
• I, personally, avoid supplements or feeds with "proprietary" formulas - i.e., they won't disclose ingredients/amounts because of ...? No one's going to steal their formula - they're all based on NRC guidelines. Non-disclosure reduces any flexibility I might have in my feeding program and can be potentially dangerous if an ingredient is not compatible with another supplement or herb I might want to use. Good examples of full disclosure include HorseTech supplements and Triple Crown feeds.
• If it works, don't fix it. If your horse is healthy, sound, has excellent hoof and coat condition, great recoveries after work and an enthusiastic attitude, figure out what you're doing right and keep doing it.
• If it's not working, take the time to figure out what and why; don't just toss the kitchen sink (or the newest magic bullet) at your horse. Part of the responsibility of owning a horse is not just throwing your hands up in the air and accepting what any one person says, but sorting through the advice, claims and advertising.

Although a commercial supplement can't balance your particular forage, there are a number of excellent supplements on the market which meet or exceed NRC minimums and don't contain potentially injurious ingredients (excess iron, excess levels of vit A or D, etc.) The best ones are mostly pretty basic - they contain flax (Omega-3), generous levels of copper (125 mg or more), zinc (360 mg or more) and vitamin E, some iodine and selenium. A healthy horse usually doesn't need anything else; additions beyond this should be customized to the individual horse/work/stress levels and their forage/feed intake.

Also, don't be misled by lengthy guaranteed analysis lists in supplements. For example, all supplement bases (flax, alfalfa, distillers grains, etc.) contain some amino acids. When nutrients such as lysine or methionine are expressed in "mg" rather than the normal "g" or "grams", that's a tip off that there is no added amino acid and the entry in the analysis is just there to impress you - "400 mg" looks like a lot more than "0.4 g". If there is actually added lysine or other amino acid, it should be listed in grams and also show up in the ingredient list.

On the other hand, some ingredients we don't want or need (iron, manganese if your hay has excessive manganese) won't show up in the "guaranteed analysis" - but you will see them listed in the ingredients.

Analyzing Hay and Feeds

(with updates 22 May 2010)

Relying on tradition or advertising to formulate your horses ration can be misleading - the only way to be certain of what your hay or feed contains is to have it analyzed by a forage laboratory. Despite traditional thought, many grass hays are more than adequate in calcium and protein for most adult horses. Sugar and starch content can vary dramatically depending on growing and cutting conditions. Some pellets and feeds do not provide adequate levels of certain nutrients and some nutrients may be in excess of desired levels (this is especially true of iron).

While most healthy horses can tolerate wide variations and imbalances, knowing what's in your feed can help provide optimum health and performance.

Where to Get Your Hay or Feed Analyzed

Dairy One Forage Lab
730 Warren Road
Ithica, NY 14850
1-800-496-3344
http://www.dairyone.com/Forage/services/Forage/forage_Price_List.htm
Submittal form
http://www.dairyone.com/Forage/services/SingleSample.pdf
Tests to request:
F-321 Forage NIR ($16) and
M-329 Wet Chemistry Minerals ($10)
or
Test # 10 – Basic ($28) plus
ESC-simple sugars ($7) and Starch ($9).
Also request Nitrates ($6) for breeding mares (and for sudan hay or for small grain hays grown in stressed conditions).

These tests will give you Protein, DE (digestible energy/Mcal), NFC, NSC (sugar and starch), major mineral, trace minerals. The NIR test also shows fat, lignen and estimated lysine. Results are reported as "%" or "ppm" and require some math to put it into a useful format.

Equine Analytical Laboratories
730 Warren Road
Ithica, NY 14850
1-877-819-4110
http://www.equi-analytical.com/default.htm
Submittal form:
http://www.equi-analytical.com/Services/Sample_information_sheet.pdf
Test to request:
(601) Equi Tech ($29, NIR)
or
(603) Trainer ($49, wet chemistry)

This is the "equine" division of Dairy One. The comparable tests cost a bit more, however the report format also shows results in grams or milligrams based on the "amount fed" you indicate on the submittal form. There is a wealth of information on the website, including a "basic" version of the NRC tables. Purchase of a hay probe from Equi-Analytical includes one "free" hay analysis.

Litchfield Analytical Services
535 Marshall St/PO Box 457
Litchfield, MI 49252
http://www.litchlab.com/
Test 4T ($26) plus NSC ($32, includes starch and sugar) Total $58
or
Equus Plus ($49, preferred)
(Selenium is $38)
No specific “submittal” form; although their website is a little difficult to navigate Litchfield was the first to provide NSC (non-structural carbohydrates: sugar and starch) testing for us and is still considered to be the “gold standard”.

A&L Canada Laboratories, Inc.
2136 Jet Stream Rd.
London, ON
Canada
Phone: (519) 457-2575 Fax: (519) 457-2664
alcanadalabs@alcanada.com
http://www.alcanada.com/indexx.php (select Submission FormsFeed Submission Form)
Request Test F2 - forage package ($25) plus DE for horses and Cushings Sugar and Starch ($25)
Total cost $50
You must write in “DE for Horses and Cushings Sugar and Starch” on the submittal form
(Selenium is $25)

Should I request the NIR or Wet Chemistry Test?

The "wet chemistry" tests are considered by many to be more precise, especially for sugar and starch (NSC). I would recommend using "wet chem" to test hay for insulin resistant (IR) mares in late pregnancy/lactation and horses with persistent or recurring laminitis.

I have found the less expensive NIR tests results to be fairly consistent with "wet chem" test results when I have had both tests run on the same sample of Bermuda hay.

How to Send Samples

The forage lab websites all have sections on taking a good hay sample. Using a hay corer will provide a more accurate representative sample for analysis. Samples should be taken from several bales (I usually sample 20+ bales in a load). The samples are mixed well in a clean bucket, then a few handfuls are placed in a "zip lock" bag – enough to fill a one-quart bag.

For pellets, obtain about a cup from several bags, mix them together, then place about a cup of the mixed samples into a "zip lock" bag.

The laboratories will provide mailers, but using a priority mailer (envelope or box for hay, a box for pellets) from the U.S. Post Office works well. Mark the plastic bag containing the sample with a description of the sample (such as "Brand X Pellets" or "Bermuda Hay"). Enclose a check for payment and the submittal form in an envelope with the complete Lab address on it. (Be sure you put your email address on the submittal form to get faster results via email.) Place the envelope and the sample in the mailer and send via Priority Mail. You should receive your results within one week by email.

What to Do With the Results

If you are comfortable with numbers, I can provide you with an Excel spreadsheet or a "paper and pencil" worksheet that you can use to calculate your supplement needs. I can also assist with in depth interpretation of the results. I use guidelines developed by Eleanor Kellon, VMD, which expand on the basic recommendations in the NRC (National Research Council) Nutritional Requirements of Horses. Current research in equine nutrition supports many changes from the last published NRC recommendations, which, in general, are "minimum requirements" rather than optimal levels.

Because minerals are synergistic and many affect the absorption and utilization of other minerals and nutrients, it is important that they be balanced. For example, most horse owners are aware that calcium should be one and a half to two times the amount of phosphorus in a horses ration; but may not know that calcium and magnesium should also be balanced.

The general balancing ratios suggested by Dr. Kellon are:

Major (Macro) Minerals:
Calcium 1-1/2 to 2 times phosphorus and magnesium
Potassium 3.3 to 10 times sodium (3.3:1 is the ideal target)

Trace (Micro) Minerals:
Iron 4 to 10 times copper (4:1 is the ideal target)
Copper generally not to exceed 4 times NRC value (based on kg of dry feed)

Update - higher levels of copper are more commonly seen now with the guidance of an experienced "balancer"
Zinc and manganese 3 times copper, with manganese lower than zinc

Update - we shoot for manganese to be 1.5 x NRC and for manganese to be at least 50% of zinc (the Cu:Mn ratio will often be lower than the previous 1:3 ratio).

There are additional considerations if manganese is high, if horse is iron overloaded or if major minerals are extremely low or high.

Healthy horses can tolerate fairly large deviations from these ratios but many circumstances call for staying close to ideal targets. Pregnancy, lactation and growth increase the requirements for protein – both amount and quality – and calories, and lessen the tolerance for imbalance, as do strenuous work and stressful conditions (climate, travel, environment). Metabolic conditions, age and illness also lower the tolerance for imbalance.

Other factors may affect balancing a ration, including long standing excesses or deficiencies, high levels of toxic minerals (molybdenum, aluminum, etc.), area water mineral levels, iron overload (which requires specific blood work to diagnose). These conditions may require addition of minerals beyond the normal "safe" levels, diluting the ration with forage from a different growing area or outright rejection of a forage or feed.

High iron levels in hay and feed are a fairly new concern in equine nutrition for adult horses (it has been studied fairly extensively in zoo animals) and it has been shown that excessive iron can be "pro-inflammatory" in some circumstances.

Nutrition Consulting

If your hay or feed falls outside acceptable parameters or if your horse has performance or health issues, it can be helpful to consult with an equine nutritionist. Good resources are extension services and university veterinary schools. Veterinarians who specialize in reproduction or equine sports medicine are more apt to be current in nutritional research. While many field vets rely on feed companies for nutritional education, many more are becoming involved in independently studying equine nutrition.

Eleanor Kellon, VMD, is an Equine Nutrition specialist and provides ration consults for race/performance horses and breeders and also provides assistance with equine metabolic problems. Dr. Kellon's basic and advanced nutrition courses can provide you with the tools and knowledge to effectively balance your own horse's ration. Dr. Kellon can be contacted through her website at http://www.drkellon.com/.

Many feed and supplement company representatives are nutritionists; however some sales representatives only have limited nutritional education. Look for one who shows how their products can meet your horse's nutritional requirements, not how they can fit your horse's requirements to their products. Because they hope you will buy their products, feed company nutritionists do not charge for services.

Some of Dr. Kellon's nutrition class graduates are available for assistance on the EC Horsekeeping group at http://pets.groups.yahoo.com/group/ECHorsekeeping/.

While I am not a certified nutritionist, I have studied equine nutrition for the past several years and can help you determine requirements and interpret feed analysis results. My emphasis is on education so the horse owner obtains the knowledge to make informed decisions about their horses' diet.

I tend to be skeptical of "magic bullet" feeds or supplements and of those whose advice is biased by the products they are trying to sell. While many feeds, supplements and herbs are useful and helpful, I feel the horse owner is responsible to understand what they are and how and why they work. You can reach me at DesertEquineBalance@gmail.com.

All feed and supplement recommendations should be reviewed with your veterinarian.


RESOURCES

NRC
National Research Council Nutritional Requirements for Horses (NRC)
Sixth Revised Edition (2007) http://books.nap.edu/catalog.php?record_id=11653

NRC Online Program

This program will calculate the requirements for your horse based on age, weight, reproductive class and activity level. It is a little more precise than using the NRC tables.

http://nrc88.nas.edu/nrh/

The NRC Nutrient Requirement Tables are available on the Equi-Analytical website at
http://www.equi-analytical.com/default.htm under the "Putting results to work" tab.

The entire previous NRC Nutritional Requirements for Horses (1989) book is available online Update - this is no longer available

Horse Journal Guide to Equine Supplements and Nutraceuticals by Eleanor M. Kellon, VMD,

http://horsebooksetc.com/products/Horse_Journal_Guide_to_Equine_Supplements_and_Nutraceuticals-879-0.html is an up to date comprehensive review of supplements and their uses.

Equine Supplementsand Nutraceuticals by Eleanor M. Kellon, VMD, (no longer in print but you might get lucky) is available from Breakthrough Publications, www.booksonhorses.com (and other sources such as Amazon) and contains excellent discussion on general and specific needs as well as information and maps concerning toxicity.

Dr. Kellon's Equine Nutrition Courses (basic through advanced) are available at http://www.drkellon.com/.

Equine Cushings Group – Equine Cushings and Insulin Resistance discussion, research and support moderated by Dr. Eleanor Kellon with reams of nutritional information.
http://groups.yahoo.com/group/EquineCushings/

EC/IRHorse.com is a sister web site to the Equine Cushings group.

Susan Evans Garlinghouse, DVM's website contains "must read" articles on nutrition for endurance horses.
http://shady-acres.com/susan/index.shtml

Kentucky Equine Research is a forerunner in equine nutrition and has a wealth of information in the articles in the Library section at http://www.ker.com/index.html?region=na



Patti Woodbury Kuvik
Desert Equine Balance
Vail, AZ
DesertEquineBalance@gmail.com