Calcium and phosphorus are the most important minerals for bone formation and maintenance in horses. These minerals must be supplied in the right amount, and also in the proper ratio, to support skeletal health. Kentucky Equine Research (KER) has conducted a number of studies to determine how well calcium and phosphorus are digested by horses.

Mineral digestibility can be expressed in two different ways. One way is as apparent digestibility. Using this calculation, the amount of a specific mineral that is recovered in the feces is subtracted from the total daily intake of that mineral. The amount that disappeared (intake minus feces) is divided by the total daily intake to produce a percentage of intake. Apparent digestibility is a fairly crude way to evaluate digestibility since it only measures the total amount of a particular nutrient in the feces. There are two possible sources of these fecal nutrients. Some of the nutrient could be the undigested residue left from the feed, but some may have actually been excreted into the digestive tract from the horse’s system, or it might have sloughed off the intestinal wall. The fecal substances that originate from inside the horse are considered endogenous in nature and they result in an underestimation of true nutrient digestibility.

To overcome the interference of endogenous losses in the determination of digestibility, a statistical procedure called a Lucas test can be utilized to calculate the estimated true digestibility of a nutrient. This test looks at a range of nutrient intakes, and the amount of nutrient that is digested is regressed against its corresponding level of intake. If there are real endogenous losses associated with a particular nutrient, then the calculated level of nutrient digested at a nutrient intake of zero will be a negative number.

Horses are generally very efficient at digesting and absorbing calcium. The upper half of the small intestine is the major site of calcium absorption. In KER’s studies to evaluate whether the concentration of other substances in the horse’s ration affects calcium digestibility, linear regressions were calculated between estimated true calcium digestibility and the concentration of several other nutrients in the horses’ rations. There were no interactions between calcium digestibility and the concentration of protein, fat, magnesium, iron, zinc, copper, or manganese. There was also no relationship between the level of iron in the diets and the true digestibility of calcium. On the other hand, calcium digestibility was negatively correlated with NDF and phosphorus concentration.

In a study to determine phosphorus absorption, results showed that phosphorus digestibility as a function of intake was much more variable than calcium digestibility, and several nutrients in the diet appeared to influence digestibility. There was no relationship between protein, calcium, or calcium:phosphorus ratio and phosphorus digestibility. True phosphorus digestibility was also negatively correlated with fiber content.

Horses that are fed grass or hay and the recommended level of a commercial horse feed will get the proper amounts of calcium and phosphorus at the correct ratio for optimal digestibility. Owners who use a mineral supplement should check with an equine nutritionist to be sure they are not causing an imbalance in the horse’s intake of these important minerals.

Joe Pagan, PhD