Q. I see that the trace minerals copper, zinc, iron, and selenium get talked about a lot, but I see others such as manganese and iodine mentioned on feed labels and supplements. I know copper and zinc are important for hoof and coat quality, iron is an important constituent of hemoglobin, and selenium is important for muscle function. But what do the other trace minerals do?
A. This is a great question, and you are right: We spend a lot of time talking about copper and zinc because they are often deficient in forage-based diets. Selenium can be too low, as well, or it might be too high, causing toxicity. However, manganese, iodine, and cobalt are also important and get far less press.
Manganese: Essential for Fat and Carbohydrate Metabolism
Manganese is essential for fat and carbohydrate metabolism. What might surprise many horse owners is that it also plays a role in the synthesis of chondroitin sulfate, which in turn is needed for cartilage formation. Many of my clients are keen on using joint supplements, most of which contain chondroitin sulfate. I’m not opposed to using (good-quality) joint supplements; however, doing so if the horse’s base diet is lacking in manganese seems unproductive. My recommendation is to ensure your horse is consuming enough manganese that he can manufacture his own chondroitin sulfate, and then from there consider supplementation.
For an 1,100-pound horse at rest, the manganese requirement per the National Research Council (NRC) is 400 milligrams per day. There’s a slight increase with work—up to 500 milligrams per day in heavy work. Depending on where hay is grown, which influences its nutritional content, a forage diet might easily meet this requirement. Most commercially fortified feeds provide manganese and should fill any shortfalls. No known cases of manganese toxicity have been identified in horses, so overconsumption is unlikely to cause problems. However, consumption of very large amounts might negatively impact phosphorus absorption.
Iodine: Required to Synthesize Certain Thyroid-Related Hormones
Most of the iodine in the horse’s body is found in the thyroid gland where it’s required for synthesizing the hormones thyroxine (T4) and triiodothyronine (T3). These hormones directly impact the horse’s metabolic rate. Diets that contain too much or too little iodine might result in hypothyroidism. Too much iodine might inhibit both the production and release of thyroid hormones. Too little results in an inability to create thyroid hormones.
The easiest way to add iodine to the horse’s diet is via iodized salt. A 1,100-pound horse’s daily iodine requirement is 3.5 milligrams per day, increasing to 4.4 milligrams when in heavy work. Feeding 2 tablespoons (or about 28 grams) of iodized table salt will add about 1.5 milligrams of iodine to the ration. Kelp-based supplements are often used as iodine sources, but you should feed these carefully because they contain generous amounts in the range of 9 milligrams per half ounce. As with manganese, many commercial feeds provide iodine in their feeds, but the level is rarely shown in the guaranteed analysis.
Cobalt: Required to Make Vitamin B12
The bacteria in the horse’s hindgut need cobalt to manufacture vitamin B12. In the form of B12, cobalt plays a role with iron and copper in the formation of red blood cells. No known cases of B12 deficiency have been reported in horses. The NRC recommends 0.05 milligrams of cobalt per kilogram of dry matter consumed. It states that horses typically receive this amount by consuming normal feedstuffs. However, research shows that when horses are supplemented with additional cobalt, their feed utilization improves. This suggests that while the supplemental cobalt might not have a direct effect for the horse, it might help hindgut microorganisms function more optimally, allowing them to better use the forage in the diet.
Again, the better-quality commercial feeds will provide small amounts of additional cobalt. Please note that cobalt has some restrictions in the racing industry. However, the amounts consumed in a typical equine diet will not be high enough to affect blood parameters.
The trace minerals, despite being required in tiny quantities each day, have a profound impact on our horse’s well-being. This is just one reason why it is important to make sure the diet you are feeding is meeting all nutrient requirements and is well-balanced. If you are unsure about any nutrient you see on feed bags or supplement containers, or if you are not sure whether nutritional needs are being met, reach out to a qualified equine nutritionist who can determine whether your horse needs additional help.