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Mineral Deficiencies
by Cynthia Chestnutt
Physical Geology
Spring 2010


                             Mineral Excess, Deficiency, and Complication in Horses
                                    Living in the Southwest United States

          Many people have gone to great lengths to address their own health issues, ensuring that they are taking vitamins, or eating foods that contain the right kinds of vitamins and minerals that their body needs. It is not surprising to know that our equine friends also have a variety of mineral needs when it comes to ensuring their optimum health. What we feed our horses has a profound effect on their growth, ability to resist disease, and in some cases to survive.

          Perhaps considered by some the most troublesome mineral complication in the horse is the development of enteroliths in the intestine of the horse. Enteroliths are hard stones, that can be from small pea size stones all the way up to stones weighing several pounds. They are extremely painful to the horse as they cause an obstruction in the intestine of the horse. During the development phase they may only partially block the intestinal path way. Many can move along the digestive tract and be expelled by the horse during normal excretion. However, if they continue to grow, they eventually block the entire intestinal pathway causing extreme and urgent colic which can only be resolved with surgery. 

          Enteroliths otherwise known as struvite are made up of magnesium, ammonium, and phosphate. Enteroliths also contain calcium, titanium, iron; aluminum and nickel however these minerals tend to compose less that 10% of the enterolith content.

Beautiful but sometimes deadly enteroliths:

          There are several hypothesis of why enteroliths are more common in the Southwestern parts of the United States. One of those is the belief that the increased popularity of feeding alfalfa to the horses in the Southwest. Alfalfa hay contains increased amounts of ammonium and magnesium. It is thought that there is an excessive amount of free ammonium released in to the large colon of the horse, and this combined with the magnesium and phosphate ions forms struvite crystals. Studies show that in fact 98% of horses with enteroliths had a diet composed of at least 50% alfalfa hay which is interestingly also high in calcium, magnesium, ammonium, and phosphate. While enteroliths do exist in other parts of the country the mineral composition of the enterolith seems to mimic that of the composition of the soil and feed in those regions.

          One important note worth asking is why is alfalfa grown in the Southwest high in magnesium? It is believed to be higher due to the majority of it being grown in serpentine soil. “A serpentine soil is derived from ultramafic rock, in particular serpentinite a rock formed by the hydration and metamorphic transformation of ultramafic rock from the Earth's mantle.” As you may already know ultramafic rocks are low in silica content and high in magnesium and iron. The results of growth in this type of soil? You guessed it: alfalfa with high levels of magnesium.

          Yet another hypothesis is that the magnesium and or calcium content of the water in conjunction with high magnesium in the alfalfa may be the culprit. In personal experience having lost a horse with an enterolith, alfalfa was removed from his diet completely and yet stones reformed. The water he drank was high in calcium. Still others believe the formation of the stone is caused by a bacterial infection that hydrolyzes urea to ammonium and raises urine pH to neutral or alkaline values. In horses urine pH is also noted to be increased and this is also how struvite is formed in dogs and humans.


          While it may seem like a good idea to avoid high magnesium, and high ammonium feeds, magnesium deficiency comes with its own host of problems. Magnesium deficiency is associated with horses that are excitable and nervous, can attribute to Insulin Resistance (where cells become resistant to the glucose uptake action of insulin) and contribute to obesity in the horse.

Photo credit:

          Insulin resistance is of particular concern as the consequences of uncontrolled insulin resistance leads to a chronic debilitation in the horse known as laminitis.  Most non-horse people know that laminitis was the debilitating last straw for Barbaro, however laminitis can also be caused by insulin resistance due to mineral deficiency.  (Some mineral excesses have also caused laminitis.) Laminitis occurs when the coffin bone of the hoof separates from the hoof wall.  The result is a failure of the structure of the hoof to support weight.  This of course results in intense pain for the horse. Often laminitis only occurs in the front feet.  The photo below shows a horse with a typical laminitis stance as he attempts to shift weight from his sore front feet to his rear.

This stance is typical of a horse suffering pain from laminitis.  The rear legs are moved forward to reduce pressue on the front hooves.

          Should the laminitis become chronic and uncontrolled distortion of the hoof capsule may result.


           According to Katy Watts of, who had dedicated her research to investigating hay composition and horses all over the country, says the horses of the Southwest are nearly all deficient in magnesium despite the high levels of alfalfa being fed. It is thought however, the competition of other minerals compete for absorption with magnesium and is therefore still deficient.  Here is the southwest excess calcium leads to magnesium deficiency despite excess magnesium in alfalfa.

           In addition, other commonly deficient minerals in the Southwest are: Copper, zinc, and selenium. According to Jim Sprinkle from the University of Arizona Agricultural and Life Sciences Extension, when Arizona rangeland soils carry low levels of trace minerals, the forage plants are also deficient in these minerals as well.

          While selenium may be deficient in Arizona, supplementation can be tricky as a little of bit of selenium is good however over supplementation can equate to the loss of mane and tail and even the sloughing of the hoof. Selenium is toxic in high doses and may result in death of the horse.

While copper deficiencies are considered rare, the symptoms can include more subtle problems in horses such as thin bones, swelling of joints, and if severe deformity in the limbs.

          An interesting symptom of zinc deficiency is brittle weak hooves. Many believe this is simply an attribute of the dry desert arid regions, however can also be caused by a deficiency of zinc, biotin, or any of the other building blocks needed for keratin formation in the hoof.

There are a variety of minerals needed by horses in Southwestern United States. Since the majority of horses in this region do not have access to fertile pasture land horse owners find that they must meet their horse’s requirements with dried hays. The downfall means horses often lack much needed minerals they could normally source foraging on their own through a variety of terrains. Even so soil levels of the ground lack many minerals horses need to support healthy functioning in which the pasture or hay is grown. The recommendation is to test all your hay that you feed, and then purchase a supplement that contains the balancing minerals your hay is deficient in.


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