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.

9 thoughts on “Calcium and Phosphorus in Horse Diets

  1. I’d like to know how to add more calcium to my horse’s diet and this article didn’t give any of that ~ any advice?

  2. My recently acquire thoroughbred is very thin and I’m giving him pellets and bran 3Xs a week. Will this deplete the calcium and phosphorus is his body and what can I give him to counteract that?

    • Hello Alida, Thanks for the question! Without knowing a lot more about your horse’s diet, such as the nutritional profile of the pellets you are feeding, and also what nutrition is coming from your hay – or at least what type of hay you are feeding – it’s tough to say what is happening in his diet. I would encourage you to read another post of ours, about the feeding of bran mashes, as those are probably not doing a lot for him, and may be hurting him in the Ca:P ratio area.
      We would suggest finding a single, fully balanced feed to put him on, rather than adding individual ingredients such as bran to his diet. Because individual ingredients tend to be strong in providing a couple of nutrients, but then weak in providing other nutrients, it’s very difficult as a horse owner to make sure the diet is balanced all the way around. For your TB, who is thin, we’d suggest looking for a feed that is high in fat – 7% or more, at least. If you want to add a rice bran supplement to increase the fat in his diet further, look for one of the commercially available types that has added calcium, to offset the lack of it in the rice bran itself.
      Hope that helps – please do let us know if you have more questions!
      Thanks ~ Gina T.

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  4. I am feeding Safe Choice to my TB and QH right now and I was wondering if they are getting the Calcium and Phosphorus that they need. They are also getting 3 to 4 flakes of hay a day. Also, about 1 table spoon of Finish Line electrolytes. So I guess to narrow down the question, about how much of C and P do they need? (both horses are on a light work schedule.)

    • Hi Emily, Thanks for the great question! The minimum recommendations for a 1000 lb horse in light work are as follows: 20 grams of Calcium per day, and 14 grams of Phosphorus per day. It takes a little calculating, but if you are feeding 5 lbs per day of SafeChoice Original, than you are giving 22.6 grams of Calcium and 15.8 grams of potassium – so they are getting more than the recommended amount (which is completely OK!) just from the SafeChoice. Now, there are lots of other factors at play here to keep in mind:
      1. How much are you actually feeding of the SafeChoice?
      2. How much is the hay providing? Would need to know type of hay, and how many pounds per day (not flakes, because flakes can vary widely in weight) of that you are feeding, to get an estimate based on some book values of hay nutrients.
      3. The requirements listed above are NRC (National Research Council) minimums – to keep a horse healthy. We believe that optimum performance comes from optimum, not minimum, nutrient levels – hence the levels in our products that provide higher amounts, when fed according to directions.

      Hope that helps! If you have more questions, please don’t hesitate to ask! Thanks ~ Gina T.

  5. What do you do when there is too much calcium in your hay and pasture? I tested mine and the calcium is high, the phosphorous is low. I ended up testing when my mare’s bloodwork showed high total calcium. The next test showed high parathyroid and normal “ionized calcium”. The mare doesn’t look sick so the lab and the vet are saying my issues are nutritional, but how do you balance that out?

    Thank you so much!

    • Hello Lynn,
      You mentioned your hay, but I was curious if you are feeding any supplements or concentrate in the mare’s diet. The calcium to phosphorus ratio can range from 1.2 to 1.5 parts calcium to 1 part phosphorus in the diet. This depends on age, activity level and gestation. Also, does your horse have free choice salt? As salt intake can increase the absorption of both potassium and calcium. My concern is that if the forage is too high in Calcium, adding a all inclusive vitamin mineral supplement will not balance the equation. You may need to consider adding another hay source to the daily ration.
      As a side note, the ideal ratio of calcium to magnesium in the diet is 2:1. You may want to ask your vet, if based on tests, adding a magnesium supplement would be beneficial to your mare.
      Thank you ~ Gayle R.

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