The first recorded use of calcium (Ca), and perhaps unknowingly so, was in the form of limestone and gypsum. Builders and construction workers used compounds in many different applications but, most interestingly perhaps, the ancient Egyptians used them in constructing the pyramids. Later, doctors recorded that gypsum was particularly useful to set broken bones; however, it was only in the 1700s that researchers discovered that Ca was a component of bones themselves.

The majority of the Ca (almost 99%) in a horse’s body is found in bones and teeth. However, Ca also has other important bodily functions. For example, it plays a role in muscle contraction, cell membrane function, blood clotting, and some enzymes’ function, as well. As such, the body must regulate blood Ca concentrations carefully. To do so, bone can act as a storage pool for extra Ca, but it is always best if a horse’s diet provides sufficient Ca.

The National Research Council’s Nutrient Requirements of Horses (NRC, 2007) recommends that a mature idle horse weighing 500 kilograms (1,100 pounds) consume 20 grams of Ca daily. This requirement increases to 30 to 40 grams per day for the same horse in light to heavy exercise. Calcium requirements for pregnant mares (weighing 500 kilograms) only increase about mid-way through pregnancy to 28 grams per day and topping out at 36 grams per day towards the end of pregnancy (NRC, 2007). Early lactating broodmares have the highest Ca intake recommendations, starting at 59 grams per day for a 500 kilograms horse, tapering off throughout lactation. Growing horses have high Ca requirements to support growth and bone health, as well (NRC, 2007).

Forages typically contain higher Ca levels than grains. However, legumes (e.g. alfalfa) on average contain more than twice the amount of Ca than do grass forages. Therefore, while grass forages might be sufficient to meet some horses’ Ca requirements (such as idle mature horses), alfalfa hay will generally provide more Ca and more closely meet growing horses’ higher Ca needs, than a grass forage.

Commercial concentrate feeds also often include a Ca supplement. Calcium can be added to a concentrate feed as either an organic (e.g. calcium-amino acid proteinate) or inorganic (e.g. calcium chloride or calcium carbonate) form. When feeding such a commercial feed in addition to forage, as per the feed manufacturer’s instructions on the bag, your horse’s Ca requirements should be met. However, you should contact an equine nutritionist or veterinarian if you have any questions regarding your horse’s Ca intake.

Mieke Holder, PhD, is an assistant research professor within the University of Kentucky’s Department of Animal and Food Sciences.

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