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Minerals for Horses: Calcium and Phosphorus

Minerals are involved in a variety of functions in the body, including enzymes, structural components, energy transfer and acid base balance. Minerals are also incorporated into vitamins, amino acids and hormones. Thus, proper mineral nutrition is vital to having a healthy horse. The minerals that are needed in the largest quantities by horses are referred to as the macro-minerals. These include calcium (Ca), phosphorous (P), magnesium (Mg), potassium (K), sodium (Na) and chloride (Cl). These minerals are needed in the diet in concentrations of g/kg or percentages, versus ppm or mg/kg of micro-minerals. This fact sheet addresses the most commonly talked about minerals that are often deficient in equine nutrition, Ca and P.


A Note of Caution

When creating diets for horses, it is important to re- member considering not only how much mineral is in the diet, but also the ratio of particular minerals in relationship to others. Minerals have very complex interactions with each other, and excesses or deficiencies of minerals can greatly affect the absorption, metabolism and excretion of others. Therefore horse owners who “tinker” too much with their horse’s diet through overzealous supplementation may be doing more harm than good for their horse.


Phosphorus Function

Phosphorus is important for bone growth and skeletal health in horses. While Ca is the major player, P makes up 14–17% of the mineral component of a horse's skeleton. But that is not all it does. Phosphorous is vital in energy transfer (ATP), DNA and RNA synthesis, cell membranes, etc. Therefore, phosphorus is an important aspect of mineral nutrition.



Osteopenia — a decrease in bone mineral density below normal. In humans, this is considered to be a precursor to osteoporosis. Horses don’t really suffer from osteoporosis.

Ossification — essentially proper bone formation replacing cartilage as the horse grows, not to be confused with calcification. While calcification is a normal process of ossification, abnormal calcification can also occur, for example the
formation of splints.

Nutritional secondary hyperparathyroidism — bone disease caused by too much P and not enough Ca. Calcium is removed from the bone, which can result in brittle, weak bones. Often manifests in shifting lameness in early stages. Previously referred to as Big Head Disease.


Environment and Phosphorus

In many situations, dietary P is adequate to meet P needs without additional supplementation. However, phosphorous runoff is an environmental concern, and not something that horse owners should ignore. Horse manure is similar to that of ruminants and has a high proportion of water soluble P. However, there is less total P as a percentage and contributes to run-off less compared to poultry and swine manure, especially in a pasture management situation. This may not eliminate the need for concern of larger stables with more concentrated numbers of animals. Overfeeding of phosphorus leads to more excretion of phosphorus into the feces, thus into the environment. High calcium diets may also lower the ability of the horse to absorb P. This does not hurt the horse, as they are still absorbing a sufficient amount to meet their needs. But when absorption from the gut is decreased, excretion increases through the feces. While horses may not be under the same scrutiny as feed yards and confinement swine facilities, the random supplementation of minerals to horses without full consideration of the diet should be avoided.


Phosphorous deficiencies and excess

Phosphorous deficiencies are typically not seen in mature horses, even when exercising. However, just like with Ca, special attention needs to be paid to the broodmares and foals, both of which are forming new bone. Inadequate P results in a slowing of the growth rate of young horses and can lead to improper bone formation. However, excess P can be more of a concern because it can inhibit Ca absorption. This interaction of minerals is why random supplementation is strongly discouraged. Prolonged conditions of excessive P can lead to the development of secondary nutritional hyperparathyroidism. Horses should not be fed more than 1% of their diet as P. Typically, this is not an issue unless the feed source is predominantly cereal grains (like wheat bran or oats), which are high in P. Conceivably, this could happen on all grass forage (i.e., not legumes) with a high grain intake containing no mineral supplement high in calcium.


Ratio of Calcium to Phosphorus

The ratio of Ca and P is always important when looking at horse rations. A ratio of two parts Ca: one part P is ideal, with a range of 1:1 to 6:1 being acceptable. Because phosphorous competes with Ca for absorption in the gut, total diets that are less than 1:1 or contain more P than Ca should be avoided. Remember to calculate the entire diet! If only one component of the diet has an inverted ratio, the total combined diet might still be fine. It is also possible to have the correct ratio, but still be deficient in these minerals because insufficient quantities are in the feed.


How much is too much?

Calcium has been fed as high as five times the horse’s requirement without any ill effects, provided that the P intake is adequate. The maximal concentration of Ca in the horse’s diet is 2%, however it would be hard to find feeds that reach that level. Alternatively, others have found that alfalfa diets (thus, higher Ca) may decrease the incidence of ulcers.


Ca and P requirements for maintenance and work

Adult horses that aren’t exercising have low calcium requirements (Table 1). There is loss of Ca in the sweat of exercising animals that is represented in the increase in requirements for work. In addition, the increase in Ca requirements for exercising horses is presumably due to an increase in bone deposition. Horses undergoing intense exercise experience an increase in bone mass, thus having a greater need for calcium. It is unlikely that light exercise, or exercise that the horse is already adapted to (essentially no change in work intensity), results in a change of calcium requirements. Additionally, most studies of calcium and exercise have focused on the young, growing horse. However, in an effort to err on the side of safety, the National Research Council recommends higher intakes of calcium. As the following tables show, maintenance horses will fairly easily meet their P requirements. In exercising horses, most of the requirements were determined using young horses
who were also concurrently experiencing bone growth and increased bone density. However, again, the P requirements for mature exercising horses are estimated to be higher, more as a margin of safety.


Table 1. Calcium and phosphorus requirements (grams/day) for adult horses at maintenance or work.

Weight (lbs) Maintenance Maintenance Light Work Light Work Moderate Work Moderate Work Heavy Work Heavy Work
900 16 11 24 14 28 17 32 23
1000 18 12 27 16 31 19 36 26
1100 20 14 29 18 34 21 39 28
1200 21 15 32 19 38 23 43 31
1300 23 16 35 21 41 24 46 34


Ca and P for Mares

For gestating mares, requirements of Ca and P increase the greatest for the 9th, 10th and 11th month of gestation, which is concurrent with the most rapid increase in fetal growth. There is also substantial fetal growth in the 7th and 8th month of gestation as well, therefore mineral requirements are elevated in this period compared to maintenance (shown as the sixth month of gestation in Table 2). Lactating mares clearly have an increase in Ca and P demand to support milk production for foal growth. Mares fed an inadequate amount of Ca actually experience a decrease in bone density, as calcium is removed from bone to supply adequate minerals for the foal through milk. In comparing Table 1 with Table 2, at least in terms of Ca, horses at light to moderate work would be considered to be comparable to gestating mares. However, demands of lactation exceed the working horses in needs of calcium. Therefore, one should either choose a feed or supplement designed to meet the needs of lactating mares and foals. After the first three months of peak lactation, the mineral demands on the mare taper off as the foal derives more nutrition from the feed it consumes.


Table 2a. Calcium and P requirements (g/d) for gestating and pregnant mares.

Weight (lbs) Month of
  6th 6th 7th-8th 7th-8th 9th-11th 9th-11th
  Ca P Ca P Ca P
900 16 11 23 16 29 21
1000 18 12 25 18 32 23
1100 20 14 27 20 35 26
1200 21 15 30 21 39 28
1300 23 16 32 23 42 30


Table 2b. Calcium and P requirements for lactating mares.

Weight (lbs) Month of
  1st 1st 2nd 2nd 3rd 3rd 4th 4th
  Ca P Ca P Ca P Ca P
900 48 31 47 31 45 29 34 21
1000 53 34 53 34 50 32 37 23
1100 58 38 58 37 55 35 41 26
1200 63 41 63 41 60 39 45 28
1300 69 44 68 44 65 42 48 30



Obviously foals get much of their Ca and P from their mothers’ milk, but as they start to ingest new feeds and taper off their reliance on the mare, a balanced diet must be ensured. Table 3 lists the approximate Ca and P requirements in grams per day of growing foals from four months until two years of age. Remember, because the foal is much smaller, and eats much less per day, the concentration of Ca and P in feed must be greater. For example, a foal which consumed 2% of its body weight in alfalfa hay that was 1% Ca would meet its requirements. If it was eating orchard grass hay that was 0.4% Ca, definitely not! There is not a large decrease in overall mineral requirements as the foal matures. But as the foal matures and reaches a larger body size, it will consume more, thus the concentration of Ca and P needed in the diet will go down. Feeding foals for rapid growth without properly balanced Ca and P levels in the diet can lead to joint disease.


Table 3.  Approximate Ca and P requirements (g/d) for growing foals based on their estimated mature weight.

Weight (lbs) Foal
  4-7 mo. 4-7 mo. 8-14 mo. 8-14 mo. 15-24 mo. 15-24 mo.
  Ca P Ca P Ca P
900 31 17 30 17 30 16
1000 35 19 34 19 33 18
1100 38 21 37 20 36 20
1200 42 23 41 22 40 22
1300 45 25 44 25 43 24


How big will the foal be?

If nutrient requirements are based on expected mature body weight, how do you know what size the young horse will ultimately be? Look at both the mare and the sire, then use an average. However, foals from maiden mares and older mares, tend to be smaller. Don’t forget that the nutrition pro- gram and environment that the dam and sire were subjected to also played a large role in their final mature weight. Also, recipient mares will have a large influence on a foal’s size due to uterine influences, in addition to their genetic dam. Thus, most farms choose larger mares for embryo transfer programs to carry donor mare’s offspring. In addition, the earlier a colt is gelded, the larger they may be at maturity. There is no firm way to know, but these guidelines can be used to estimate mature body weight.


What's in a feed

Before formulating diets, it might be helpful to look at some typical feed stuffs used to create diets for horses. The following values are estimates only. Remember that forages grown on mineral-deficient soils may have lower values. Typi- cally, most grains are going to be higher in P than Ca. This should not pose a problem because concentrates should be fed to provide the extra energy or protein the horse needs, rather than making up the majority of their diet. The exception in Table 4 is beet pulp, which isn’t a cereal grain at all. When looking at some typical hays, legumes (the alfalfa and red clover) provide much greater amounts of Ca than do grass hays. This makes them ideal choices for broodmares and foals. Even most grass hays have Ca in the correct proportion to P, which makes feeding an imbalanced ratio hard to do. Note however, that orchard grass may be an exception. Now, let’s put some of these numbers together. For simplicity, we will work with a generic 1,100-pound horse, then compare nutrients with the table values for different classes of horses.


Table 4. Common concentrates fed to horses. All percentages are on a dry matter basis.

Concentrate % Ca % P
Beet pulp 0.89 0.09
Barley 0.06 0.3
Cracked corn 0.04 0.30
Rolled oats 0.11 0.40
Rice bran 0.07 1.78
Wheat bran 0.13 1.18


Table 5. Common forages fed to horses. All hays are assumed to be harvested at the mid-bloom stage. Remember: soil type and stage of maturity can alter hay nutrient content.

Forage % Ca % P
Coastal Bermuda 0.19 0.27
Alfalfa 1.27 0.24
Brome grass 0.29 0.28
Red Clover 1.38 0.24
Fescue 0.41 0.30
Orchard grass 0.27 0.34
Timothy 0.48 0.23


The first example will feed this horse two different diets, one solely Coastal Bermuda grass hay, and one of alfalfa at 2% of his body weight per day. All values above are on a dry matter basis.


First, determine how much the horse will eat.
1,100 pounds x .02 = 22 pounds

As Ca and P requirements are in grams, convert pounds to
22 pounds x 1 kg/2.24 pounds = 9.82 kg of hay

The horse will eat 9.82 kg of hay per day.

For the grass hay, multiply the amount fed by the percentages of Ca and P in that hay.

9.82 kg x 0.0019 = 0.0187 kg of Ca


Then, convert kg into grams.
0.0187 kg x 1000 g/kg = 18.7 g Ca

Now for P.

9.82 kg x 0.0027 x 1000 g/kg = 26.5 g P

Finally, calculate your Ca to P ratio.
18.7 g Ca/26.5 g P = 0.71 to 1


So, what does this tell us? First, a mineral supplement should be supplied for the horses to avoid the inverted Ca to P ratio. Alternatively, some legume hay could be added to its diet. When looking at simply meeting the requirements of the 1,100-pound horse, the diet is deficient in Ca if it is a working horse and certainly very low if that was all that was fed to a gestating or lactating mare.

Now, what if alfalfa hay was fed instead? Use the same calculations, but insert the new percentage of Ca and P for alfalfa.


9.82 kg x 0.0127 x 1000 g/kg =124 g Ca

Now for P.
9.82 kg x 0.0024 x 1000 g/kg = 23 g P

Finally, calculate the Ca to P ratio.
124 g Ca/23 g P = 5.4:1


Now the Ca to P ratio is more desirable. Looking at the horses’ requirements, the Ca requirement has been more than met for all classes of mature horses, and is adequate for P for all working horses except those in heavy work. For mares, the diet is adequate until the last part of gestation and through lactation. Thus, broodmares should be fed a better quality diet than other horses.


Lastly, what happens if we decide to add 6 pounds of oats to this 1,100-pound horse’s diet? Note: This is done solely for the purpose of calculations. There should always be some rationalization for why concentrate is added to a horse’s diet. In this example, there is no information regarding class of horse or what its body condition score is.


Begin with the red clover.
9.82 kg x 0.0138 x 1000 g/kg = 135 g Ca


Now for P.
9.82 kg x 0.0024 x 1000 g/kg = 23 g P


Now calculate the contribution from oats.
6 pounds x 1 kg/2.24 pounds = 2.7 kg oats

2.7 kg x 0.0011 Ca x 1000 g/kg = 3 g Ca
2.7 kg x 0.0040 P x 1000 g/kg = 11 g P


Add the two values together for hay and oats.
135 g Ca from hay + 3 g Ca from oats = 138 g Ca
23 g P from hay + 11 g P = 34 g P


Calculate the ratio
138 g Ca/34 g P = 4.1:1


The ratio of Ca to P is appropriate.


Now again, compare across the classes of horses. Calcium is adequate for all classes, and P requirements are met for all horses except the lactating mares.


Lastly, add the oats to the orchard grass hay.

9.82 kg x 0.0027 x 1000 g/kg = 26.5 g Ca


Now for P.

9.82 kg x 0.0034 x 1000 g/kg = 33.4 g P


The oat values will remain the same as above.


Add the two values together for hay and oats
27 g Ca from hay + 3 g Ca from oats = 30 g Ca
33 g P from hay + 11 g P = 44 g P


Calculate ratio of Ca to P.
30 g Ca /44 g P = 0.68:1


This ratio is inverted and should be avoided.


Remember that these diets are a simple exercise in calculating the contribution of calcium and phosphorous from different feed sources. These are not recommendations for actual diets, as no attempt was made to adjust amount fed, supplements added or appropriate concentrates selected.

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