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Calcium and Phosphorus in a Horse’s Diet

If you have ever been involved in selecting feed or developing a horse’s ration, then you have probably done your research and found calcium and phosphorus to be minerals that are spoken about often.  Calcium and phosphorus are two of the most abundant minerals in the horse’s body.  These two minerals work together to aid in the development and strengthening of bones, as well as muscle function.  You have probably asked yourself, “So how much should your horse be receiving and what is the correct ratio?”


Correct calcium to phosphorus ratio should be no less than 1:1.  The ideal ratio is 2:1 and certainly no more than 4:1. The incorrect calcium to phosphorus ratio can negatively affect how other minerals are absorbed in the body.  While a calcium deficiency is rare, it can happen and present itself in the horse in several ways.  Horses with a calcium deficiency can show lameness, weak bones, and low quality growth and performance.  A phosphorus deficiency can show up as muscle weakness and trembling. If too much calcium or phosphorus is in the diet, several problems can occur. Too much of both of these minerals can cause problems, such as soft tissue becoming like bone.  You can tell is your horse is getting too much phosphorus by similar symptoms of being calcium deficient such as:  weakened bones, lameness and low quality growth and performance.  Over supplementation of other minerals, such as copper and zinc, can negatively affect the horse as well.  Too much of other minerals can disturb how effective other minerals in the body are absorbed.  Most horses that are eating quality forage and a commercial grain mix should not need any kind of supplementation.  If you are concerned about your hay quality, contact your local feed supplier or extension office to get it tested.  Excel Equine® does this service for free for its customers!  If the hay test comes back and after looking at it combined with whatever grain is being fed, your horse’s ration is higher in phosphorus than calcium, a calcium supplement could be necessary.  Pregnant or lactating mares, foals, or horses in heavy performance competition should be fed a commercial grain mix that has higher levels of calcium and phosphorus available.  Grass hay alone is often not an adequate calcium source for broodmares and foals.  A mixture of legume and grass hay is ideal for lactating mares, foals, or horses in heavy competition.


So as a horse owner, what is the best management strategy for determining if you are feeding the correct calcium and phosphorus levels?  First, I would get my hay tested.  Then, I would make sure I was buying a commercial grain mix that is fortified with the correct calcium and phosphorus ratio.  After getting hay test back, I would consult the nutritionist or rep for the feed company that I buy my feed from to see what their opinion is on what grain I should be feeding after looking at hay results.  I would ask if they recommend changing hay companies if a higher quality is needed.  If feeding broodmares and foals, I would go ahead and give them legume hay in addition to grass hay.  This kind of hay has the added protein and calcium often needed by these animals.  Also…be careful when feeding bran mashes.  Bran is high in phosphorus, so if this is given to the horse every day for a long period of time, phosphorus toxicity might become present.


The biggest take away here is that you really cannot determine if you are feeding the right amount of these two minerals by just looking at a feed tag.  You need to look at the entire ration…this includes hay and grain.  Pay special attention to hay, as this should make up the majority of your horse’s diet.  So get your hay tested and then work with equine nutrition professionals from there.