Phosphorus (P) is another mineral that is essential to your horse's health.

Like calcium, this mineral is a large part of your horse's bones. Nearly 14% of your horse's bones are P.

However, being a part of bone is not its major role in the equine body. It is extensively involved in the production of adenosine diphosphate (ADP) and adenosine triphosphate (ATP), both of which are major energy units for your horse's cells.

P also plays a major role in cell membranes, as it is an important part of phospholipids, which make up the membranes. This important mineral can also be found in other important areas, such as:

  • nucleic acids (they make up DNA)
  • phosphoproteins (proteins bound together with phosphoric acid)

The absorption of dietary P depends on many factors, including:

  • other parts of your horse's diet
  • the type of P
  • the amount of P
  • your horse's age
  • amount of calcium in the diet
  • temperature (horses in warmer climates have better absorption

Like with calcium, it appears that your horse can somewhat adjust the amount of P absorbed. In situation of high intake, a lower percentage will be absorbed, ans with lower intake levels, higher percentages will be absorbed.

The average P absorption is estimated to be 30-50%. For adult horses, the absorption rate is assumed to be 35%, while 45% is used for growing horses and lactating mares (more on this below).

However, besides the factors above, there are a number of other minerals that can affect P absorption.

High intakes of calcium severely limit the absorption of P, due to the fact that these two minerals compete directly for absorption in the small intestine. Therefore, the more calcium that is absorbed, the less phosphorus can be absorbed, and vice versa. This is one of the many reasons that the calcium:phosphorus ratio is so important in horse diets.

The addition of extra sodium chloride to your horse's diet can increase absorption of P to approximately 30-60%.

Oxalates do not necessarily reduce absorption, but they do decrease retention abilities in the horse.

Dietary Sources

The main source of P in plants, phytate-phosphorus, is not easily digested by horses. There is some phytate (the enzyme that can break this source down) in the cecum, but not enough to provide adequate amounts of P.

For this reason, P is often supplemented in the equine diet through inorganic phosphates that are added to commercial feeds.

The reason that P absorption is assumed to be lower for mature horses is because these horses are often eating mostly forages, which have mostly phytate phosphorus.

However, growing horses and lactating mares are often supplemented with commercial feeds, which contain inorganic sources that are easily digestible. Therefore, their absorption rates are assumed to be higher, at 45%.


Excess P in the diet will cause a decrease in calcium absorption (since they compete for absorption) and a chronic calcium deficiency.

Because it causes calcium deficiency, excess P will also cause nutritional secondary hyperparathyroidism (NSH).

Phosphorus Deficiency

A deficiency of P in the diet causes a number of bone problems.

In growing horses it causes problems similar to rickets, which is caused by a vitamin D deficiency.

In mature horses P deficiency causes osteomalacia, or softening of the bones.

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