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.

11 thoughts on “Calcium and Phosphorus Levels in Horse Diets

  1. this is probably a question, but where can i read about what hays are best for what horses (ie-exercised alot vs sedentary) and how much hay to feed per day… thank you for your time.
    ~karen on behalf of roo and marley and missed! 🙂

  2. We have bulk bags of super-phosphate stored in a shed and our three horses were really getting stuck into it. I removed them immediately (they did not get too much). We make our own feeds – predominantly whole oats and supplements with plenty of pasture hay (rye-grass and clover)Are my horses deficient in phosphate to go to the extreme of eating fertilizer, and would adding more oats to their diet supply their needs?

    • Hello Georgie – Good question. The fact that the horses were seeking out the super-phosphate suggests that they are marginal or deficient in some minerals in the diet.

      I would suggest the following:
      1. Make sure the horses have salt available free choice. Salt deficiency is one of the first things that will cause horses to eat unusual substances.
      2. Diet may be short on phosphorus or other minerals. Without having forage analysis or supplement analysis, tough to predict. Best bet is to offer a good 2:1 Calcium:phosphorus loose mineral that also contains some copper, zinc, manganese and magnesium. A general non-medicated livestock mineral would work just fine.

      I would not expect the super-phosphate to be harmful in small quantity for limited time, but not recommended due to potential for fluoride or other metal content.

      Best wishes,
      Roy J.

  3. Our 18 year old Paso Fino mare has been living on beet pulp and alfalfa pellets for several years as she is allergic to any dust in hay. She gets some grain but she is on the heavy side so just a taste is added. She is on pasture about half the time in summer. Will adding mineralized salt to her diet improve her P absorbtion levels enough to compensate for the calcium in the alfalfa?

    • Hi Janet, Thanks for the question. Most mineralized salt products do not contain enough phosphorus to make much of a difference, and generally they also have both calcium and phosphorus, so it would also contribute to both the amounts of calcium and phosphorus and not really solve your issue. It really depends on the guaranteed analysis of the mineralized salt. Some guarantee max calcium .85% and only 0.02% phosphorus.

      Mature horses can have a maximum 6:1 ratio of calcium to phosphorus but a 2:1 ratio is ideal. A diet consisting of only alfalfa could be a 4:1 or 5:1 ratio or up to 6:1 ratio, but it really depends on where the alfalfa is grown and what the alfalfa test in nutrients which can be different from one cutting to the next or stage of maturity. Alfalfa that is late bloom will have a lower level of calcium compared to early bloom alfalfa.

      For your horse, we wouldn’t be too concerned, but the best recommendation would be to first do the math on your horse’s diet – gather the tag from the beet pulp, the tag from the alfalfa pellets (the manufacturer may add some phosphorus to the formulation to help offset the high calcium level), and also the tag from the grain you mentioned. Then put the numbers for calcium, and then for phosphorus, in to the following formula, and compare the two results to see what the ratio is:

      ((Pounds of beet pulp x percent calcium in beet pulp) + (Pounds of grain x percent calcium in grain) + (Pounds of alfalfa pellets x percent calcium in alfalfa pellets) / Total pounds of feed = Percent Calcium in Total Diet

      Then, if the imbalance is big, find a phosphorus supplement to compliment alfalfa only diet. Also, many researchers believe that if adequate vitamin D is in the diet than the ratio of calcium to phosphorus is less critical. Sufficient vitamin D assists in absorption and utilization of calcium and phosphorus and horses get vitamin D naturally from sunlight. Vitamin D deficiency is really only a concern for stalled horses.

      We hope this helps – please let us know if you have more questions!
      Thanks ~ Gina T.

  4. We will be starting a barley fodder room in the next few weeks. According to the nutritional information available online about sprouted barley, the calcium/phosphorus is way off. Do you have any recommendations of what I could add to either the fodder or their diet which would fix the ratio?

    • Hi Kim, Thanks for checking in. It’s great that you are looking at the nutritional levels of your feed sources. The first thing to do though, before you begin supplementing, is to evaluate your total diet, not just the single ingredient. In your case, for example, if your hay source were primarily alfalfa, you might actually come out alright on the total dietary level and ratio of calcium & phosphorus, because alfalfa can be very high in calcium.

      So, we would suggest having all of your feedstuffs analyzed if you haven’t already, and then look at the diet as a whole. If, after all of that, your calcium:phosphorus ratio is still inverted and you are providing more phosphorus than calcium, then look for a basic mineral supplement – many of them include calcium, and may be at levels that are enough to fix your situation.

      Hope that helps, let us know if you have more questions! Thanks ~ Gina T.

  5. Hi, my horse is on an oats and rye/clover hay diet and recently had a splint crated showing demineralization of the splint bone. I am concerned he is lacking calcium due to the oats?? I have introduced small amounts of Lucerne hay and also a white millet/linsee porridge mix ( mainly for some amino acids), can you please recommend how I balance his diet? He is Arabian warmblood 16.3hh 9yo doing moderate eventing, and requires as much energy input as possible!!!

    • Hello Tanya, Thanks for the question! We would suggest switching to a balanced, fortified product and dropping the oats and millet/linseed mix. That way, you know everything is at the levels he needs, without having to mix or try to balance on your own. Especially with the activity level and bone/joint stress of a sport like eventing!

      We would suggest feeding a product such as SafeChoice Original or SafeChoice Perform – they will provide all the amino acids, vitamins, and minerals (including calcium & phosphorus!) in the amounts and ratios he needs. Remember, it’s not just how much of all the nutrients, but also the ratios that is important. You can feed all the calcium you want, but if its not balanced with the amount of phosphorus, you will still have trouble.

      Hope this helps – if you have any other questions, please let us know! Thanks ~ Gina T.

  6. Here is what Carey Williams, phd, Rutgers University said about Milliet for Horses:

    Millet isn’t typically fed to horses in the United States. Even though it has high levels of silica, due to its hard outer shell it needs to be finely crushed in order for the horse to process the grain. The horse will not benefit from eating it whole.

    There is another concern with feeding millet to horses, however. It contains a glucoside (a plant-derived chemical derived from sugars) called setarian, which causes considerable kidney irritation and excessive urination in horses. This can be especially problematic in horses with existing kidney problems.

    Feeding an adequate level of calcium in the correct ratio with phosphorus (2:1) will provide adequate nutrition for young horses establishing proper bone growth

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