It is only appropriate to follow last month’s column on calcium (Ca) with its sidekick, phosphorus (P). Phosphorus is the second-most abundant mineral in the horse’s body; about 80% of it is found in horses’ teeth and skeleton.

Calcium and phosphorus are very closely linked, because Ca combines with P to form hydroxylapatite, found in bone and teeth. A dietary deficiency or excess of either one can interfere with the other’s absorption and utilization. As such, when evaluating a diet, ensuring that the P and Ca requirements are met is important, but making sure the horse is consuming an appropriate ratio of Ca to P in the total diet is paramount.

Your horse’s total dietary Ca:P ratio should not drop below 1.1:1 (National Research Council’s Nutrient Requirements of Horses, 2007; NRC). Typically, unfortified grains (e.g., oats or corn) naturally contain more P than Ca. Additionally, if you live in an area geographically rich in P (such as Central Kentucky), forages also tend to contain higher amounts of P. In some cases, a grass forage’s P content can even exceed the Ca content. In cases where the Ca:P ratio drops below 1.1:1, additional Ca will be needed in the diet to improve the total dietary Ca to P ratio. You can accomplish this by adding legumes or Ca-fortified concentrates.

The NRC recommends 14 grams of P (and 20 grams of Ca) per day for a mature, idle horse that weighs about 1,100 pounds (500 kilograms). If you live in a high-P area, your horse will likely be consuming more P than he or she needs, even from forage alone. The horse’s body will absorb all the P that it needs from the digested feed in the hindgut and excrete the excess in manure.

It’s important to note that excessive P in manure is also concerning, due to its impact on water quality. Pastures with poor manure management practices in place (or even poorly managed manure storage facilities) that are located near water bodies can result in nutrient runoff into waterways. The water’s resulting nutrient enrichment can contribute to algae blooms, some of which produce toxins.

Your veterinarian or equine nutritionist can provide advice if you have any questions regarding your horse’s dietary P (or other nutrients) content.

Mieke Holder, PhD, is an assistant research professor and Ashley Fowler, PhD, is a post-doctoral scholar within the University of Kentucky’s Department of Animal and Food Sciences.

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