Calcium and Phosphorus Levels in Horse Diets

Where do calcium and phosphorus come from, and what can you as a horse owner do to balance the diet?

As always, first examine your hay source.  Legumes are better sources of calcium than are the grass hays, but neither legumes nor grass hays are very good sources of phosphorus.  Legumes tend to have a significantly higher Ca:P ratio, with many coming in at around 6:1.  Grass hays, however will have a Ca:P ratio that is about 1:1 and often are even shown to be slightly lower in calcium than phosphorus.  The best way to know what is in your hay is to have it tested.  Check with your local Extension Office for places to get your feed tested.

Next, take a look at your grain source.  For cereal grains (oats, corn, etc.) the reverse is true in terms of calcium and phosphorus relative to roughages. The cereal grains are poor sources of calcium, but are moderate sources of phosphorus.  Oil seed meals, by-product feedstuffs from the distilling and brewing industry, and protein concentrates are often low to moderate in calcium but are excellent sources of phosphorus.  Commercially available feeds should have levels and ratios in balance, but don’t forget to check any supplements you are using. If you are feeding a straight grain, such as oats, or if you are using feed from local grain elevator, which do not always guarantee calcium and phosphorus levels, it is a good idea to have your grain tested along with your hay.

From these two points, it is easy to see where a horse on a grass hay and oats diet – still one of the most common equine diets in use – could suddenly be suffering from an calcium and/or phosphorus deficiency.  Sadly, a deficiency of either of these two minerals is not something that will show up quickly in a manner that is visible to the horse owner.  Years of this imbalanced diet will lead to demineralization and weakening of the bone structure, which can result in increased susceptibility to lameness and injuries.

Calcium and Phosphorus in Horse Diets

Horses are more likely to suffer from a lack of calcium (Ca) and phosphorus (P) than of any other minerals.  Considering the detrimental effect on bone development that a deficiency of these two minerals can have, every horse owner should investigate where their horse’s diet is at in regards to them.

One of the first things any animal science student learns in their Animal Nutrition 101 course is that an animal must always supply calcium in a higher amount than phosphorus, no matter what the species.  The reason for this is that phosphorus will bind up calcium in the animal, making calcium unavailable for use in bones, teeth, and other areas it is needed for.  If there is more phosphorus than calcium available to the horse, then the phosphorus will start to pull calcium away from wherever in the body it can, including the all-important bone structure! 

The reverse situation has very little in the way of detrimental effects, however.  As long as there is more calcium than phosphorus, there is very little risk of getting in to a calcium toxicity situation.   In horses, the ideal ratio is to have between 1.2:1 and 2:1 Ca:P.  Studies have shown up to a 6:1 Ca:P ratio not having an ill effect on horses.

One other note of interest in regards to calcium: it is also essential for proper tubule formation and cell attachment in the hoof wall of the horse. These parts of the hoof wall provide resiliency to the hoof and attach the wall to the sole of the foot, and inadequate calcium or poor calcium-to-phosphorus ratios can cause brittle hooves or tenderness on one or more feet. Proper balance of calcium and the addition of protein to horse rations can help produce strong hooves with well-defined tubules.