Calcium (Ca) is probably one of the most well-known minerals in the equine diet. This is due mostly to its interaction with phosphorus in the Ca:P ratio and the big impacts that ratio can have on growing horses (and adult horses too!).

Many areas in the equine body require Ca. One of the most important is bone, which is approximately 35% Ca in the horse.

However, Ca also has other roles in the equine body:

~ It plays a role in muscle contraction, by activating potassium ion channels .

~ It plays a role in cell membrane functions, allowing certain things to pass through the membrane while restricting others.

~ It regulates many enzymes in the equine body.

Though Ca has many functions in the equine body, approximately 99% of it is found in bone.

Calcium Absorption

One of the unique things about Ca is that the horse can adjust how much is absorbed from the diet.

If blood concentrations are low, a higher percentage of the dietary Ca will be absorbed. In low concentration situations, a hormone is also secreted that allows bone to reabsorb Ca more efficiently. The kidneys are also affected by this hormone, increasing their ability to reabsorb Ca.

Likewise, if blood concentrations are high, less dietary Ca will be absorbed.

Under normal conditions (Ca intake not too high or too low), approximately 50% of dietary Ca will be absorbed and used by the body. However, in growing horses this number can be as high as 70%.

Ca absorption can be highly influenced by other minerals. Excess phosphorus in the diet greatly reduces the Ca absorption, because both minerals are absorbed by the same area in the small they compete for absorption. However, increased magnesium in the diet increases Ca absorption.

Another substance that influences absorption is oxalate. Oxalate dramatically decreases absorption, and an inclusion of just 1% of oxalic acid in your horse's diet can decrease Ca absorption by more than 60%.

Dietary Sources

Legumes, such as alfalfa and clover, contain relatively high amounts of Ca -- over 1% on a dry matter basis. In comparison, grass hays contain roughly 0.6% Ca on a dry matter basis.

Cereal grains are also low in Ca, with corn being about 0.04% Ca on a dry matter basis and whole oats coming in at 0.07% Ca.

Some other common feedstuffs and their Ca concentrations:

Feedstuff % Ca
(on dry matter basis)
Barley 0.06%
Beet Pulp (molasses) 0.89%
Beet Pulp (no molasses) 0.91%
Corn 0.04%
Oats (rolled) 0.11%
Rice Bran 0.07%
Soybean Meal 0.39%

Calcium Deficiency

Due to the large amount of Ca in bones, they make a great storage place for Ca in the horse's body. However, because they are the primary storage area, Ca is also readily removed from bone when there is a dietary deficiency. As a result, prolonged Ca deficiency can result in weakened bones in the horse.

Ca deficiency can also lead to a shifting lameness in horses.

In growing horses, Ca deficiency can have severe consequences. Deficiency often leads to problems with growing bones, leading to diseases such as osteopenia, which is characterized by crooked long bones and enlarged joints. These problems are due to improper mineralization of the osteoid tissue.

Metabolic bone disease (MBD), which is a general term for many bone disorders caused by different diseases, can also be caused by Ca deficiency. In fact, a survey of Thoroughbred farms found that dietary Ca intake was directly related to the severity of metabolic bone disease. Those farms that fed diets low in Ca had more severe MBD while those that fed diets higher in Ca had a much lower incidence of MBD.

Calcium Toxicity

It has been thought in the past that osteochondrosis may be caused by excess Ca in the diet. However, in many studies, Ca has been fed at high levels (sometimes exceeding 5 times the recommended daily intake) with no ill effects, provided that the calcium:phosphorus ratio remained correct.

So it would appear that an imbalanced Ca:P ratio is of much more concern than excess dietary Ca intake.

Calcium:Phosphorus Ratio

The ideal Ca:P ratio in horse diets is the range of 1:1 to 2.5:1. If the ratio falls below 1:1, Ca absorption will be impaired, causing a Ca deficiency.

Ratios as high as 6:1 may be acceptable in growing horses, provided that the amount of phosphorus in the diet is adequate.

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