Major Minerals: A Balancing Act

By Susan Kauffmann
With special thanks to Dr. Eleanor Kellon

            Most horse owners are aware that minerals are an important part of a horse’s diet.  Some of us try to address this with mineralized salt blocks, supplements or “balanced” feeds, while others believe that a horse will get all the minerals it needs from pasture and/or good quality hay.  Who is right?  Well, like everything else in the horse world, it depends on who you talk to. 
The gold standard for the mineral requirements of horses has long been the National Research Council guidelines, but some researchers and equine nutritionists now feel that these guidelines, published in 1989, are out of date and in need of adjustment.  Some recommend feeding as much as two times the NRC requirements or even more, yet many prefer to stick with the current recommendations, pointing out that there is simply not enough data to support radical changes. 
            Things become even more confusing when you start to look at the interactions of various minerals, the ratios required to keep them all balanced, the bioavailability of different mineral forms, and so on.  Where does this leave the average horse owner?  Should you be supplementing your horse’s diet?  If so, what should you be using, and how much?
            The first step in determining whether or not your horse needs mineral supplementation is figuring out what minerals he is getting in his diet.  The only real way to do this is to have your forage and other feed sources analyzed, then supplement accordingly under the guidance of a well qualified equine nutritionist.  Short of that, there are certain generalizations one can gather about common feeds and forages that are useful when formulating a diet, though far from perfect. 
In addition, it makes sense for all horse owners to try to gain at least a basic understanding of what a horse’s mineral requirements are and how the various minerals affect each other in the horse’s body.  Because a complete examination of these topics would fill a library, this article will focus on the minerals most commonly deficient or out of balance in the average horse’s diet:  calcium, phosphorus, magnesium, copper, zinc and selenium.
            A good place to start is by taking a look at the roles of calcium (Ca) and phosphorus (P) in the horse’s body.  These minerals interact closely with each other and are involved in many important bodily processes, the most significant of which is the development and maintenance of bones.  Another function of calcium is proper contraction of the muscles, including the heart, while phosphorus is needed for kidney function, cell repair and the metabolism of sugars, carbohydrates and fats. 
In order to perform properly, calcium and phosphorus must be present not only in certain amounts, but in a certain ratio to each other – ideally with the Ca:P balance at approximately 2:1.  Most nutritionists agree that healthy adult horses can tolerate a ratio as low as 1:1 and as high as 5:1, but you never want to see the ratio reversed (1:2, for example), as too much phosphorus interferes with the body’s ability to absorb and utilize calcium.  This can cause a serious calcium deficiency, even if the total amount of calcium being consumed is within the NRC guidelines or even higher.  Ultimately, this makes the Ca:P ratio even more important than the total amounts of each mineral.
               Though the importance of the Ca:P ratio has been known for many years, horses are still more likely to suffer from a deficiency or imbalance of calcium and phosphorus than of any other minerals.   In adult horses, this can lead to fractures or mysterious, transient lameness, while growing horses lacking the correct amounts of calcium and phosphorus can suffer from developmental orthopedic diseases such as osteochondrosis and rickets (osteomalacia).  These problems occur because the bones are the body’s storehouse of calcium, so if there isn’t enough of this mineral coming in through the diet, the body will take calcium from the bones to meet other metabolic needs.  This “leeching” of calcium is why horses can end up with weak, brittle bones – or malformed bones and joints in growing horses – if they don’t get adequate amounts of calcium and phosphorus in the right proportion.  The fact that the body will leech calcium from the bones is also why serum testing is a poor indicator of Ca:P status, as there may appear to be plenty of calcium in the blood, but it could have come from the bones, not the diet.
               The most common problem – too much phosphorus in proportion to calcium – can be caused by feeding large amounts of grain or regularly feeding wheat bran or non-calcium balanced rice bran.   Both grains and grain by-products like bran are typically high in phosphorus and low in calcium, which is why it is often better to feed a balanced, commercial grain feed than plain grain.   In the past, it was not uncommon for horses to suffer from “big head” or “bran disease” (nutritional secondary hyperparathyroidism), though this is not frequently seen today.  In this condition, a severely inverted Ca:P ratio causes the concentration of blood calcium to drop, which triggers the release of parathyroid hormone. This hormone then releases calcium from the bone in order to maintain an adequate concentration of blood calcium.   When extensive calcium is removed from the head bones of the horse, the amount of fibrous connective tissue increases, causing the head to actually increase in size.  The head appears puffed or swollen, hence the name "big head disease."  Fortunately, that problem is seen far less often these days, due in part to the awareness modern feed manufacturers have of the importance of the Ca:P ratio. 
                Many horse owners, however, try to do “the balancing act” on their own, and unfortunately,  this can actually make matters worse instead of better.  For example, some owners, convinced that grain is necessary but aware of the high phosphorus levels, try to counter this by feeding large portions of alfalfa, which is known to be high in calcium.  However, it is easy to go overboard with alfalfa, which can skew your Ca:P ratio in the opposite direction.   This can cause a number of problems, particularly for very hard working or endurance horses, as excess calcium affects the body’s production of calcium-regulating hormones, which are then not able to function properly when called upon during extended, strenuous exercise.  The end result for these horses can ironically be insufficient plasma calcium levels, which can lead to metabolic-related disturbances including synchronous diaphragmatic flutter ("thumps").    Excessive calcium can also lead to magnesium deficiency, weakened bones due to changes in the pattern of mineral deposition caused by the secondary magnesium deficiency, and has been indicated in epiphysitis, which is an inflammation of the growth plates. 
            Many nutritionists now recommend that the majority of a horse’s diet should consist of grass hay, which at least usually falls within the range of 5:1 to 1:1 in its Ca:P ratio.  If you do need to increase your horse’s calcium intake (important for pregnant or lactating mares as well as weanlings and growing horses), or you want your horse to have more caloric energy than grass hay provides, you may want to consider adding some beet pulp to the daily rations.  Beet pulp, which is high in calcium, is gaining favor in many corners as the feed of choice to up the calcium and/or energy content in a horse’s diet, as you avoid the possible high protein issues of alfalfa, while at the same time getting approximately the same energy of oats without the glycemic spike of a grain.
               Another mineral that is frequently deficient and must be considered when looking to establish a healthy diet is magnesium (Mg).  Like calcium and phosphorus, magnesium is an important component of bone, but magnesium is also crucial for hundreds of non-bone related functions in the horse.   Magnesium dependent processes include regulation of the insulin response to maintain normal blood sugar levels; proper nerve functioning; protein synthesis for the production and repair of muscle and hoof tissues; normal contracture of cardiac and skeletal muscle, maintaining circulation and preventing blood clots. 
Since magnesium, calcium and phosphorus can interfere with each other’s absorption and utilization, it is important to keep an approximately 2:1:1 ratio of Ca:P:Mg.    
               Magnesium deficiency is thought to be very common in horses, often because of diets too high in calcium and/or phosphorus, but also because magnesium has been consistently depleted in our soils, a situation made worse by the use of potassium and phosphorus laden fertilizers, which alter the ability of plants to uptake magnesium.  Because of this, horses fed only grass or hay without any supplementation are often magnesium deficient, and because magnesium is lost during exercise, hard-working horses can become magnesium deficient if care is not taken.
                 Signs of magnesium deficiency include nervousness, muscle tremors or cramp, poor tolerance for work and in extreme cases, convulsions and even death.   Because of magnesium’s key involvement in nerve function, horses that are commonly described as “high strung” or “spooky” may actually just be suffering from a magnesium deficiency, which is why many products touted as “calming” agents for horses are magnesium based.   Horses that get overly excitable, hyperactive or difficult to handle in the Spring may similarly be magnesium deficient, due to the particularly low levels of magnesium in rapidly growing grasses.  Such horses frequently do calm down when magnesium levels are brought back to normal levels through supplementation.
              Surplus magnesium is excreted in the urine, so excessive magnesium consumption is not generally dangerous in and of itself, although there is some evidence that it may be a factor in the formation of enteroliths (intestinal stones).  The main problem with too much magnesium is its effect on the body’s ability to absorb and use calcium and phosphorus.  Too much magnesium may therefore cause a deficiency of calcium and/or phosphorus, even if the amounts of calcium and phosphorus in the diet are within the recommended levels.  This is why it is so important to consider the ratios of all three of these minerals when attempting to supplement any of them. 
            Yet another ratio that should be considered is that between copper (Cu) and zinc(Z).  Copper is important for the normal development of cartilage, the conversion of cartilage to bone, skin and hair pigmentation, the strength and elasticity of blood vessels, the formation of hemoglobin, nerve conductivity and coordination, prevention of anemia, fertility health and a number of enzymatic functions.  Some of the problems associated with copper deficiency are low fertility in mares, contracted tendons, epiphysitis and other developmental orthopedic disorders in growing horses, anemia, dull and/or “bleached” coats, and a loss of pigment in the skin around the eyes and muzzles.  Some people believe that it can also make horses more vulnerable to bacterial or fungal skin problems like mud fever and rain scald.
            Like copper, zinc is important for many enzymatic functions, acting as both an activator and a co-factor.  There are also more than 200 zinc containing proteins, so it is not surprising that zinc is necessary for healthy skin, bone, connective tissue and hoof growth.  A deficiency of zinc can lead to reproductive problems, a variety of metabolic disorders, as well as bone and cartilage problems, brittle or crumbly hooves, dull coats and dry, flaking skin.
               It is not uncommon for the total amounts of both copper and zinc to be deficient in our horses’ diets, and this can be exacerbated by an imbalance between the two, particularly if there is far more zinc than copper.  It is recommended that the Cu:Z ratio be maintained at a maximum of 1:3, meaning that if your zinc level is more than three times your copper level, you would need to supplement copper.  However, high levels of other minerals such as molybdenum and sulfur can complicate this balance, as these are also known to “tie up” copper, making it unavailable to the body.  In fact, many minerals can influence and interfere with the body’s use of many other minerals, which is one of the reasons why mineral balancing is so complicated.
Minerals are also closely linked to certain vitamins, requiring adequate amounts of those vitamins to be correctly processed and utilized.  One frequently deficient mineral that is well known for its interaction with a particular vitamin is selenium (Se), which works with vitamin-E as part of the cellular antioxidant defense system.  Both nutrients play key roles in the production of enzymes that work to neutralize free radicals that form as byproducts of metabolic activity and as a result of exposure to environmental contaminants. Recent research has also revealed a probable role of selenium in converting thyroxine (T4) to triiodothyronine (T3), which is the tissue active form of thyroid hormone.  It is therefore possible that some of the clinical symptoms often attributed to hypothyroidism in horses are, instead, a result of selenium deficiency. 
               What the horse owner needs to keep in mind about selenium is the fact that while it is deficient enough in many areas to require supplementation, excess selenium is a dangerous poison, and it doesn’t take much to overdose even a big animal like a horse.  Chronic selenium poisoning is an extremely painful condition which can cause separation of the hoof and severe lameness.  In the worst cases, the whole hoof may become deformed and even fall off, requiring that the horse be euthanized.  Acute selenium toxicity, which is sometimes called “blind staggers”, is characterized by apparent blindness, head pressing, abdominal pain, diarrhea, perspiration, increased heart and respiration rates, and lethargy.  Because of the low margin of error in supplementation of selenium, it is particularly important to get professional guidance in regards to this mineral – and to make sure you are not supplementing selenium in an area that already has too much of it, or feeding multiple products that each contain added selenium.
                 If you choose to supplement any individual minerals, be aware that the minerals available at your local feed store are compounds, not pure, elemental minerals, so the amount you need to add must be calculated based on the percentage of elemental mineral in the compound.  For example, calcium carbonate contains 40% calcium, so if you wanted to add five grams of calcium to the diet, you would need to supplement with 12.5 grams of calcium carbonate.  You may also wish to examine the benefits of using organic (chelated) mineral forms as part of your supplementation program, as research suggests that organic minerals have greater bioavailability – meaning that they are more easily absorbed and utilized – than inorganic forms.  Hard working horses in particular seem to benefit from the inclusion of organic minerals in their diets.
                It is also extremely important to keep in mind that adding any individual minerals without taking into account possible toxicity issues, as well as the complex interactions of the entire mineral spectrum, may cause serious harm or potentially dangerous imbalances, and it is therefore recommended that you only supplement individual minerals under the guidance of a qualified equine nutritionist or veterinarian.
                 The table below illustrates the NRC guidelines for the minerals discussed in this article (click on it to bring up all the columns, as they don't all show in this view).  Your nutritionist or veterinarian may very well recommend levels higher than those listed here, particularly for growing horses and pregnant or lactating mares.

            If you are interested in finding out more about minerals and supplementation, we recommend the book Equine Supplements & Neutraceuticals:  A Guide to Peak Health and Performance, by noted veterinarian and equine nutritional expert, Dr. Eleanor Kellon.  A good online source of information is The Equine Cushings Group at:  Though the site is primarily aimed at owners of horses with metabolic disorders, its files section and archives contain a great deal of excellent information on minerals, mineral balancing, and general nutrition for horses.

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